Common Illnesses

Betta Gillfriend care rules: one betta per tank. Mirror or flare practice: one 2-minute session a day, with at least 2 rest days a week. Train: up to 10 minutes a week.

This is the biggest section in the guide, because when your betta is unwell you need clear, calm information fast. Please read the first part, "Before you treat anything", even if you jump straight to a specific illness.

Warning: This guide is not a substitute for a veterinarian. Fish medicine is a real veterinary speciality. Many conditions look alike, and some can only be confirmed with a microscope, a culture, or an X-ray. Treatments, and especially doses, vary between products, countries, water conditions and individual fish. Wherever possible, work with an aquatic vet. We deliberately describe treatments in general terms and do not give doses. Always follow your vet's instructions or the product label exactly.

Before you treat anything

Step 1: Test the water first

Merck Veterinary Manual names poor water quality as the most common cause of environmental disease in fish, and it also weakens the immune system so infections can take hold. Before reaching for medicine, test ammonia, nitrite, nitrate, pH and temperature. Many "illnesses" improve once the water is fixed, and medicating fish in bad water rarely works.

Step 2: Look closely and write it down

• Use a torch (flashlight) in a darkened room. Shining it from the side makes dust, spots and film much easier to see.
• Note: breathing rate, fin position, colour, appetite, faeces, swimming, any spots, fuzz, film, lumps, wounds or swelling.
• Take clear photos and a short video. These help a vet enormously.
• Check the symptom lookup table.

Step 3: Isolate if needed

If your betta shares a tank, or you need to medicate, move them to a hospital tank.

Step 4: Contact an aquatic vet

• Find a fish vet: the American Association of Fish Veterinarians has a "Find a Fish Vet" map for North America at fishvets.org, and the World Aquatic Veterinary Medical Association (WAVMA) also has a directory. Some vets offer remote consultations.
• In many places, the most effective fish medications, such as antibiotics and praziquantel in the US, legally need a vet's involvement (a valid veterinarian–client–patient relationship).
• What to bring: Merck suggests bringing the sick fish if possible, plus a separate water sample of at least 500 mL from the tank in a clean, sealed bottle or bag with no air bubbles, kept cool. Bring your test results, photos, and a list of everything you've added to the tank (including any treatments).
• If a fish has died: a freshly dead fish can still help a vet diagnose the problem, which may protect your other fish. Chill it in a sealed bag in the fridge (do not freeze it) and contact the vet quickly. Some parasites, including gill flukes, leave the body soon after death, so speed matters.

Medication safety and the hospital tank

Golden rules for medicating

1. Diagnose before you medicate. Merck and UF/IFAS warn against "shotgun" treatment: using several medicines at once, or treating "just in case". It can kill fish, breed drug-resistant bacteria, and hide the real problem.
2. Don't mix medications unless a vet tells you to. Combinations can be toxic, and you won't know which one worked or harmed.
3. Remove activated carbon (and other chemical filter media) during treatment. Carbon absorbs many medications and stops them working. After treatment is finished, put fresh carbon back in, along with water changes, to help remove what's left (as UF/IFAS advises for treated water).
4. Measure your water volume accurately. Real water volume is often less than the tank's label (because of substrate and decor). Overdosing and underdosing are both dangerous.
5. Medications can damage your biofilter. Antibiotics and some other treatments harm beneficial bacteria, so test ammonia and nitrite daily during and after treatment.
6. Boost oxygen. Many medications (formalin especially) reduce oxygen, and sick fish, especially those with gill disease, need more of it. Add an air stone and lower the water level slightly to increase surface movement.
7. Copper is risky. Merck notes copper becomes more toxic in soft water with low alkalinity (KH), and it is deadly to snails and shrimp. Use it only with guidance.
8. Follow the full course a vet prescribes, even if your betta looks better.
9. Watch closely during treatment. If your betta rolls over, gasps, loses balance, or suddenly gets worse, move them into clean, matched water at once and call your vet.
10. Remove invertebrates. Snails and shrimp are often killed by medicines that fish tolerate.

Warning: Wear gloves when handling sick fish or tank water, and wash your hands afterwards. Some fish bacteria, notably Mycobacterium, can infect people through cuts. Merck advises that people with weakened immune systems avoid contact with affected tanks.

Setting up a hospital tank

A hospital tank (or quarantine tank) is one of the best investments a betta keeper can make. Merck describes a simple, inexpensive setup.

Using it:

• Fill it with water from the main tank (if the main tank is healthy) or fresh dechlorinated water matched for temperature and pH.
• Test ammonia daily, and do small, temperature-matched water changes. Re-dose medication after a water change only as the vet or label directs.
• When finished, disinfect the tank and all equipment, rinse thoroughly, and store dry (Merck). Ask your vet which disinfectant suits the illness. Merck notes that bleach does not reliably kill Mycobacterium, and recommends 70% ethanol or phenolic disinfectants for that.

First-aid kit

Have these ready before you need them:

• Liquid test kit: ammonia, nitrite, nitrate, pH (and ideally GH/KH)
• Spare heater and thermometer
• Water conditioner that treats chlorine and chloramine (and ideally binds ammonia)
• Hospital tank, sponge filter (pre-seeded), air pump, air stone and tubing
• Dedicated nets, bucket, siphon and turkey baster or pipette for the hospital tank
• Clean containers for water samples (500 mL or more)
• Bright torch / flashlight and a magnifying glass
• Disposable gloves
• A notebook, or use the app's health log, plus phone camera for photos and video
• Contact details for your nearest aquatic vet (find one now, at fishvets.org)
• Indian almond leaves (optional, supportive only)
• Aquarium salt or plain non-iodised salt only if your vet has advised its use. Don't use it routinely.
• Fresh activated carbon, to remove medications after treatment

Tip: We don't recommend keeping a cupboard full of medications "just in case". Medications expire, and guessing which to use is how fish get shotgun-treated. Get a diagnosis, then get the right medicine.

Daily and weekly health checklist

Every day (2 minutes)

• Temperature is in range (76–81°F / 24–27°C)
• Betta comes to greet you and eats eagerly
• Breathing is calm; both gill covers move evenly
• Fins are open (or held normally for plakats) and edges look clean
• No spots, dust, fuzz, film, lumps, wounds or swelling
• Swimming is level and controlled; no floating, sinking or listing
• Belly looks normal: not bloated, not sunken; no pinecone scales
• Eyes are clear and not bulging
• Uneaten food removed; equipment running

Every week

• Test ammonia, nitrite, nitrate and pH, and log the results
• Partial water change (10–25%) with gravel vacuum
• Look over your betta in good light, from the side and from above, for subtle changes
• Check fin edges for slow changes and compare photos from last week
• Check decor and plants for sharp edges or rot
• Check the filter flow and the heater
• Check food is fresh (open container is less than about 2 months old)

Symptom lookup table

Use this as a starting point, not a diagnosis. Many conditions share signs, and two can happen at once. Always test water first.

Conditions A to Z (grouped by type)

Each entry follows the same pattern: what it is, signs (early and late), look-alikes, first steps and treatment approaches, prevention, and when to see a vet.

Water and environment problems

Ammonia, nitrite and nitrate poisoning

What it is and why it happens Ammonia (from waste and rotting food) and nitrite (made from ammonia by bacteria) are toxic. They build up in uncycled tanks ("new tank syndrome"), after overfeeding, when a filter is cleaned too hard or stops, after medications kill the biofilter, or in small tanks with too few water changes. Ammonia burns the gills and skin. Merck notes un-ionised ammonia as low as 0.05 mg/L damages gill tissue. Nitrite stops blood carrying oxygen ("brown blood disease"). High nitrate over a long time is a chronic stressor.

Signs

• Early: clamped fins, lethargy, poor appetite, hanging near the surface, faster breathing, stress stripes.
• Late: gasping at the surface, red or inflamed gills, red streaks in fins, cloudy eyes, lying on the bottom, sudden death. Damaged skin and gills invite fin rot, fungus and columnaris.

Look-alikes: gill flukes, velvet, columnaris of the gills, low oxygen, chlorine exposure. Your test kit is what tells them apart. If ammonia or nitrite is detectable, fix that first.

First steps and treatment approaches

1. Test ammonia, nitrite, nitrate and pH.
2. Do a large, temperature-matched, dechlorinated water change. Merck recommends 50% or more when ammonia is high. If the pH is low and ammonia very high (old tank syndrome), Merck advises small daily changes instead, to avoid a pH shock that turns ammonium into toxic ammonia.
3. Use a conditioner that binds ammonia as a short-term help.
4. Stop or greatly reduce feeding for a day or two.
5. For nitrite, Merck notes that adding chloride (salt) protects fish against nitrite toxicity. Ask your vet whether this is appropriate and at what level.
6. Find the cause: an uncycled tank, overfeeding, a dead snail, filter failure, or medication.
7. Keep testing daily until ammonia and nitrite stay at 0.

Prevention: cycle before adding fish (Tank setup), feed small meals, do regular water changes, rinse filter media only in tank water, and test weekly.

When to see a vet: if your betta is still gasping or not eating a day after water quality is corrected, or if there are visible gill, skin or fin injuries.

Temperature shock

What it is and why it happens Bettas are cold-blooded, so their immune system, digestion and activity depend on water temperature. Sudden changes, such as a heater failing or sticking on, a cold water change, a power cut, or moving between tanks, are stressful. Merck notes that fish immune responses are inhibited at temperatures outside their natural range, and gives a general guideline of changing temperature by no more than about 1°C (or 1°F) per hour.

Signs

• Cold: sluggish, lying on the bottom, clamped fins, poor appetite, pale colour, slow digestion and bloating. Cold, stressed fish are more prone to ich and fungus.
• Too hot: rapid breathing, gasping at the surface (warm water holds less oxygen), frantic swimming, then collapse.
• Sudden shock: the fish may lie on its side, dart erratically, or go into shock shortly after a water change.

Look-alikes: chlorine/chloramine exposure or pH shock after a water change, ammonia poisoning.

First steps

1. Check the thermometer, not just the heater's dial.
2. Bring the temperature back into range slowly, by about 1°C an hour.
3. Keep lights low and the tank quiet. Don't feed until behaviour is normal.
4. For a heater stuck "on", unplug it and replace it. For a power cut, insulate the tank with blankets (keep the lid on). See Power outages for a full step-by-step plan.

Prevention: use a reliable heater sized for the tank; keep a spare; check the thermometer daily; always temperature-match new water; keep the tank away from windows, radiators and air-conditioning vents.

When to see a vet: if your betta doesn't recover within a day, or if signs of infection appear in the following week.

Lethargy

What it is: lethargy (low energy) is a symptom, not a disease. A research colony lists lethargy, clamped fins and losing interest in food as the three most common signs of a sick betta.

Common causes

• Water that's too cold, or poor water quality (most common, so test first)
• Infections and parasites (gill flukes, velvet, internal parasites)
• Overfeeding and constipation
• Old age
• Strong current wearing out a long-finned fish
• Stress from tank mates, reflections or a busy room

First steps: test water and temperature; look closely for other signs (use the symptom table); reduce flow; check for bullying. Note whether he still eats. A betta that rests more but eats eagerly is less worrying than one that won't eat.

When to see a vet: lethargy for more than a couple of days with good water, any loss of appetite for more than 2–3 days, or lethargy combined with breathing changes, swelling, or visible marks.

Parasites

Gill flukes (Dactylogyrus and Gyrodactylus)

If you've lost a betta to gill flukes, we're so sorry. Please know that this is one of the hardest fish illnesses there is to spot and to treat, even for professionals. Flukes are tiny, hide in the gills, often show only vague signs until they're numerous, and can arrive on fish that look perfectly healthy. It is very common for keepers to do everything right and still face them. This section is here to help you feel more prepared, not to look back with blame.

What gill flukes are

"Gill flukes" and "skin flukes" are the everyday names for monogeneans: tiny parasitic flatworms (most less than a millimetre long) that cling to fish with a hooked attachment organ called a haptor. They feed on mucus and the cells of the skin, fins and gills. (UF/IFAS points out that, scientifically, they aren't true flukes, but the name has stuck.) The two groups betta keepers hear about most:

Monogeneans have a direct life cycle: they pass straight from fish to fish, with no other host needed. That means they can build up inside a single aquarium.

The life cycle, and why one treatment isn't enough

1. Adults live on the gills or skin, feeding and damaging the tissue.
2. Egg-laying flukes release eggs. The eggs have sticky or trailing threads that catch in gill mucus, filter media, gravel and plants.
3. Larvae (called oncomiracidia) hatch from the eggs and swim off to find a fish.
4. Live-bearing flukes skip the egg stage. Their young can attach straight away or drift to another fish.
5. Speed: UF/IFAS notes that at around 22–25°C (72–75°F), the whole cycle can take only a few days. Bettas are kept a little warmer than that, so things can move fast.

Warning: UF/IFAS states that the eggs of all monogeneans are impervious to treatment. Treatments kill adults and newly hatched larvae, but not eggs waiting in the tank. This is why treatment has to be prolonged or repeated, timed so that each new wave of hatching larvae is killed before it can mature and lay more eggs. Stopping too soon is one of the main reasons flukes "come back".

How flukes arrive

• New fish, including fish that look completely healthy. UF/IFAS explains that some fish have enough immunity to keep flukes at low numbers and show no signs, yet still carry them and pass them to other fish.
• Water from a store bag or another tank.
• Shared equipment: nets, siphons, buckets, decorations or filter media moved between tanks. Fluke eggs are sticky and can ride along.
• Plants and decor from tanks that held fish, because eggs and larvae can be carried in water or debris.
• Stress makes it worse. UF/IFAS notes that stressors (poor water quality, aggressive tank mates, poor nutrition, crowding) weaken the immune response. In studies, stress hormones (corticosteroids) actually increased fluke reproduction.

Early and subtle signs

Flukes are invisible to the naked eye, so you'll be watching for behaviour and breathing. Early signs are easy to miss or put down to "a bad day":

• One gill cover held open, flared, or redder than the other. Betta keepers commonly report this one-sided look with gill flukes. It's a useful early clue, though not proof.
• Faster or laboured breathing, with gill covers working hard even at rest
• Gasping or "piping" at the surface, or spending more time near the top
• Flashing: suddenly darting or rubbing a gill or side against decor or gravel
• Pale or swollen gills, or extra mucus (a greyish film or stringy mucus)
• Lethargy: resting more, sitting in corners, hiding
• Clamped fins
• Loss of appetite, or spitting food out
• Patchy colour or scale loss where flukes are feeding (more with skin flukes)

UF/IFAS lists lethargy, swimming near the surface, clamped fins, sitting in corners, poor appetite, flashing, scale loss and colour changes as signs, and with heavy gill infestations, swollen and pale gills, increased breathing rate, lower tolerance of low oxygen, and piping (gulping air at the surface). Merck adds excess mucus and flared gill covers.

Later signs: severe breathing distress, refusing all food, weight loss, open sores or secondary infection (UF/IFAS notes that secondary bacterial and water-mould infections are common on fluke-damaged tissue), and sudden death. Merck notes monogeneans can cause very high losses in captive fish.

Telling flukes apart from look-alikes

Tip: The most important first step is a water test. If ammonia and nitrite are 0, and there's no visible dust, spots or patches, but your betta is breathing hard and flashing, gill flukes move high up the list, and it's time to call an aquatic vet. Only a microscope can confirm flukes. A vet takes a tiny gill or skin sample (a biopsy or mucus scrape), often without harming the fish. UF/IFAS also notes flukes leave a fish soon after it dies, so if you've had a loss, a vet needs a freshly dead fish to check.

Treatment approaches (in general terms)

Treatment for flukes is genuinely difficult, and your vet is your best partner. Here's what aquatic vet sources describe, so you know what to expect and can have an informed conversation:

• Praziquantel is described by UF/IFAS as the treatment of choice for monogeneans in ornamental and aquarium fish, and Merck lists it for both skin and gill flukes. It specifically targets flatworms and is safer for the fish than many alternatives. Important cautions from UF/IFAS:
  • In the US it is not FDA-approved for fish and is used "extra-label" under a vet's care (a valid veterinarian–client–patient relationship).
  • It's typically used as a prolonged bath over a couple of weeks, or in repeated rounds, to catch larvae as they hatch from treatment-resistant eggs.
  • Praziquantel doesn't dissolve well in water. If the powder is not prepared properly, it can severely irritate the gills.
  • Research shows it may break down quickly in established aquariums, which can make treatment less effective, one reason treating in a clean hospital tank under vet guidance helps.
  • Water changes during treatment may need re-dosing to keep the right concentration. Ask your vet.
  • Treated water should pass through activated carbon before it's disposed of.
• Formalin is used to control flukes, but it removes oxygen from the water, needs vigorous aeration, and UF/IFAS notes sick fish tolerate it poorly. It's generally a vet-guided option.
• Hydrogen peroxide and potassium permanganate have been used against flukes, but UF/IFAS warns they need extreme caution: the effective doses are close to doses that harm fish.
• Salt gives variable results against freshwater flukes. UF/IFAS describes a study where a small salt increase actually led to more gyrodactylids, because the fish made extra mucus that the parasites fed on. Salt shouldn't be relied on as a fluke cure.
• Dips (short baths) can reduce parasite numbers, but UF/IFAS notes sticky eggs survive them.
• Organophosphates, which have been used in the past, are considered less effective than praziquantel (Merck) and are hazardous. Avoid them.

Supportive care during treatment

• Oxygen, oxygen, oxygen. Damaged gills can't take in oxygen well. Add an air stone, keep surface movement up, keep the water clean, and avoid overheating. Keep the temperature steady within the normal range (76–81°F / 24–27°C). Avoid pushing it to the top of the range during gill disease unless your vet advises it, because warmer water holds less oxygen. (This is our suggestion, not a tested protocol.)
• Pristine water: test daily, especially as medications can affect the biofilter.
• Low stress: dim lights, a hiding place, no flare sessions, no tank changes.
• Easy food: small amounts of tempting food. Remove leftovers quickly.
• Recheck: UF/IFAS advises re-examining fish 1–2 weeks after treatment to see whether it needs repeating.

Warning: Please don't combine fluke medications with other treatments, use several products "to be safe", or guess at praziquantel doses from forums. Gills damaged by flukes are fragile, and the wrong product, dose or preparation can do more harm than the parasites. An aquatic vet can confirm the diagnosis, choose the right drug and dose for your betta's water, and advise on timing repeat treatments.

Prevention

• Quarantine every new fish for at least 4 weeks (UF/IFAS; Merck suggests at least 30 days for many fish) in a separate tank, with its own nets and equipment. Watch breathing, gills and behaviour closely, especially in the first 1–3 weeks when stress from shipping often makes hidden infections show. See Quarantine.
• Ask about prophylactic (preventive) treatment. Merck notes that preventive praziquantel during quarantine is often sensible for some fish groups, and UF/IFAS recommends taking steps to prevent flukes whenever new fish are introduced, even when the new fish appear healthy. Whether a preventive course makes sense for your new betta is a good question for your vet. A quarantine-period gill check (biopsy) by a vet is the gold standard.
• Never share nets, siphons, buckets or decor between tanks without disinfecting and drying them. Ideally, give each tank its own.
• Don't add store water to your tank.
• Quarantine or rinse new plants from tanks that held fish. Buying plants grown without fish (e.g. tissue-culture plants) avoids the risk.
• Keep stress low: good water, good food, no bullying tank mates. Stress helps flukes multiply.

After losing a fish to flukes: before you add new fish

Losing a betta, especially after fighting hard to save them, is heartbreaking. Take the time you need. When you feel ready, here is how to give your next fish the best start:

1. Remove anything that's left, including any other fish, to a separate tank if they need monitoring or treatment, under vet advice.
2. Understand the tank may still hold eggs and larvae. UF/IFAS explains that apart from emptying, drying and disinfecting a tank, it is very difficult to completely eliminate monogeneans once they're in a system.
3. Choose your reset:
   • Full reset (most certain): empty the tank; throw out or thoroughly disinfect substrate, decor and filter media; disinfect the tank, heater, filter, nets and buckets (ask your vet which disinfectant to use, then rinse very thoroughly and dechlorinate); let everything dry completely; then set up fresh and cycle again.
   • Fishless period: monogeneans have a direct life cycle and need a fish host to survive. Leaving the tank with no fish for a while means larvae that hatch from leftover eggs have no host to find. There's no single agreed waiting time for home aquariums, so ask your vet what's appropriate for your tank and temperature. If in doubt, the full reset is the safer choice.
4. Discard or disinfect live plants from the tank, or keep them in a separate fish-free container for some time first.
5. Quarantine your next betta for at least 4 weeks, ideally with a vet check, before it goes into the main tank.
6. Keep a record of what happened (dates, signs, treatments, results). If a vet examined your fish, their findings can guide prevention next time.

Tip: Some keepers find it comforting to set up the cleaned tank, cycle it, and plant it over several weeks. It gives you time to grieve, and your next betta a safe, mature home to arrive into.

When to see a vet: as soon as you suspect flukes, especially fast breathing or a flared gill with normal water tests. Flukes can escalate quickly, and early, correctly prepared treatment gives the best chance.

Ich (white spot)

What it is and why it happens Ichthyophthirius multifiliis is a single-celled parasite. The feeding stage (trophont) burrows under the skin and gills, where it's protected from medication. When mature, it drops off and forms a cyst (tomont) on surfaces, divides into hundreds of new parasites, and releases free-swimming theronts that seek a new fish. Only the free-swimming stage can be killed by treatment. UF/IFAS notes that new fish can be carriers with no signs, and stress from shipping often triggers disease 1–3 weeks after arrival.

Signs

• Early: flashing, clamped fins, darker colour, less appetite, and a few tiny white dots on fins or body.
• Late: many white spots "like salt grains", rapid breathing, lethargy and death. UF/IFAS warns that by the time spots are easy to see, the fish is already quite sick, and if ich is only on the gills, there may be no visible spots at all.

Look-alikes: velvet (much finer, gold dust), lymphocystis (larger, cauliflower-like lumps), other skin parasites, and tiny air bubbles stuck to the skin (these disappear).

First steps and treatment approaches

• Move to a hospital tank if possible, or treat the whole tank (ich will be in the tank too).
• UF/IFAS notes that at warm temperatures (75–79°F), the life cycle takes about 3–6 days, so treatments are applied daily, with at least 3–5 treatments. Your betta's normal temperature is already in this range.
• Vet sources list formalin and copper (with copper's cautions), and UF/IFAS describes salt in a prolonged bath as effective in small systems if the fish tolerates it. Discuss with your vet or follow product labels exactly.
• Vacuum the substrate during treatment to remove cysts.
• Keep treating until well after the last spot disappears, as the label or your vet directs.
• Add aeration; remove carbon during treatment.

Prevention: quarantine new fish; keep temperature stable; avoid stress.

When to see a vet: if spots persist after a full course, if breathing is badly affected, or if you're unsure whether it's ich.

Velvet

What it is and why it happens Velvet is caused by Piscinoodinium, a parasitic dinoflagellate (a kind of single-celled alga-like organism) that attaches to and invades the skin and gills. Merck notes it can cause high mortality, and a research betta colony lists velvet among the most common betta diseases. It's often brought in by new fish and flares up with stress.

Signs

• Early: clamped fins, flashing, loss of appetite, lethargy, faster breathing. A faint gold, rusty or yellowish "dust" or a thin velvety film (Merck calls it rust or gold dust disease). It's very hard to see in normal light. Shine a torch from the side in a dim room.
• Late: thick dusty coating, peeling skin, severe gasping, death. If it's only on the gills, the dust may not be visible at all (Merck).

Look-alikes: ich (bigger, distinct white spots; Merck describes velvet spots as smaller and harder to see), gill flukes (no dust), natural metallic or iridescent scales (these shine evenly over the scales), columnaris.

First steps and treatment approaches

• Act quickly. The research colony found velvet could be treated successfully when caught early.
• Merck lists copper sulfate and, for non-food fish, chloroquine (a prescription drug) as treatments. Copper needs careful dosing, testing, and enough water hardness and alkalinity to be safe.
• Many keepers dim the lights during treatment, because the parasite uses light. Ask your vet about this as supportive care.
• Strong aeration and pristine water. Remove carbon during treatment.

Prevention: quarantine; avoid stress; keep temperature stable.

When to see a vet: as soon as you suspect velvet. It can kill quickly.

Internal parasites and Camallanus

What they are

• Spironucleus (previously called Hexamita): a flagellate that lives in the intestine. Merck lists bettas among susceptible fish, with weight loss and deaths of young fish as signs. Keepers often notice white, stringy faeces.
• Camallanus: a red roundworm that lives in the lower intestine. Merck notes the worms can be seen as red threads protruding from the vent ("red worm disease"). Copepods (such as Cyclops) and similar small crustaceans can carry it, so wild-caught live foods are a risk.
• Tapeworms and other worms also occur in aquarium fish.

Signs

• Early: eating well but not gaining weight, or slowly getting thinner; pale, stringy faeces; slightly swollen or sunken belly.
• Late: a "pinched", sunken belly with a large-looking head, lethargy, visible worms at the vent, death.

Look-alikes: constipation (swollen, not thin), mycobacteriosis (chronic wasting), old age, tumours, starvation from poor diet.

First steps and treatment approaches: a vet can look at a fresh faecal sample under the microscope. Merck lists metronidazole (for Spironucleus, non-food fish only) and fenbendazole or levamisole (for Camallanus), and praziquantel for tapeworms. These are usually given in food or as baths under vet direction. Because worms and eggs can remain in the tank, vets often advise repeat treatment and thorough cleaning.

Prevention: buy live foods only from cultured sources (or use frozen); quarantine; good hygiene.

When to see a vet: any visible worms, ongoing weight loss, or white stringy faeces lasting more than a few days.

Bacterial and fungal infections

Fin rot

What it is and why it happens Fin rot is a bacterial infection of the fins, usually from common water bacteria such as Aeromonas and Pseudomonas (Merck), that takes hold when fins are damaged or the fish is stressed. Poor water quality is the usual trigger. A research colony found fin rot is among the most common betta diseases, and that fish older than about 1.5 years seemed more prone to it.

Signs

• Early: fin edges look ragged, frayed or slightly discoloured (white, black, brown or red edges). The fins may look thinner.
• Late: fins shrink back towards the body; edges look "melted", pink or bloody; rot reaches the fin base and body. The research colony describes "wasted, jagged fins" with pink necrosis.

Look-alikes: fin biting (clean, sudden gaps, often in a pattern, without discoloured edges), snag injuries, columnaris (often faster, with white or yellow patches), natural crowntail rays, and in some dark or marble bettas, colour shifts at the edges.

First steps and treatment approaches

1. Test and fix water quality, with extra clean water changes (keeping temperature and pH stable).
2. Remove sharp decor and check for fin-nipping tank mates.
3. Mild early cases often stop and fins regrow with clean water alone. Regrowth is often clear or white at first, which is normal.
4. Spreading or severe cases may need antibiotics. Merck recommends sensitivity testing to choose the right one, and UF/IFAS warns against random antibiotic use. Antibiotics can harm your filter bacteria, so use a hospital tank.

Prevention: clean, stable water; no sharp decor; gentle flow; compatible tank mates.

When to see a vet: if rot reaches the body, spreads quickly, or doesn't stop within about a week of excellent water.

Fin and tail biting

What it is and why it happens Some bettas bite their own fins, especially long-finned males (halfmoons and others with very heavy fins). Stress, boredom, fins that are too heavy to swim with comfortably, strong flow, and reflections are commonly suspected triggers. It's widely reported by keepers, but there's little formal research.

Signs: chunks or clean-edged gaps appear suddenly, often overnight; fins may shorten in an even pattern; sometimes you'll actually see your betta turning to bite. No white, black or melted edges, at least at first.

Look-alikes: fin rot (ragged, discoloured, progressive edges), tank-mate nipping, snag injuries.

First steps

• Rule out tank mates and sharp decor.
• Reduce stressors: lower flow, cover reflective glass, add plants and resting spots, and stop or reduce flare sessions.
• Add enrichment (see Enrichment and flare training).
• Keep water very clean so bitten fins don't get infected.

Prevention: a calm, planted, enriched tank with gentle flow and resting places near the surface.

When to see a vet: if bites become infected (discoloured or spreading edges), or if biting continues despite changes.

Columnaris

What it is and why it happens Columnaris is caused by the bacterium Flavobacterium columnare, which is common in warm-water fish. Strains vary from mild to highly aggressive. Merck notes outbreaks are linked to water quality problems and stress. Mortality can be sudden, and by the time it's noticed it may be hard to control.

Signs

• Early: a pale, greyish or white patch on the body, fins or around the mouth; frayed fin edges; a slimy or cottony look.
• Classic: Merck notes columnaris is sometimes called "saddleback" or "cottonmouth" disease, for the pale band that can form across the back and the cotton-like growth around the mouth (often mistaken for fungus). Lesions may be slimy or cotton-like; on the gills, vets may find creamy or yellowish material.
• Late: ulcers, rapid spread, gill damage with gasping, death, sometimes within a day or two in aggressive strains.

Look-alikes: true fungus (Saprolegnia, fluffier, usually on existing wounds), fin rot, ich, velvet, gill flukes (no visible patches). See Mouth fungus vs columnaris.

First steps and treatment approaches

• Fix water quality and increase oxygen immediately.
• Merck notes that early treatment with potassium permanganate can help, but if the disease isn't caught early, antibiotics are generally needed (chosen by a vet, ideally after testing). These treatments are easy to get wrong, so vet guidance is strongly recommended.
• Treat in a hospital tank; disinfect equipment afterwards.

Prevention: good water, low stress, quarantine, careful handling.

When to see a vet: promptly. Columnaris can move faster than almost any other betta illness.

Fungal infection

What it is and why it happens Most "fungus" on aquarium fish is water mould, such as Saprolegnia and relatives (Oomycota, fungus-like organisms). Water mould usually grows on damaged tissue: injuries, fin rot, parasite damage, or ulcers. Merck links it to low temperatures and poor sanitation.

Signs: white, grey or brownish cotton-wool tufts that may look fluffy in the water; often on a wound, fin edge or the eye. Untreated, it spreads and can be fatal.

Look-alikes: columnaris (often flatter, slimy, yellowish, saddle-shaped), lymphocystis (firm lumps), uneaten food stuck to the fish, and excess mucus.

First steps and treatment approaches

• Find and fix the cause of the original wound (water, injury, parasites).
• Clean, warm, stable water.
• Antifungal treatments are available. Follow labels or vet guidance, and treat in a hospital tank.
• Have a vet check whether it's truly fungus or columnaris, as they're treated differently.

Prevention: keep water clean and warm (76–81°F / 24–27°C), handle gently, prevent injuries.

When to see a vet: if it spreads, involves the gills or eyes, or returns.

Mouth fungus vs columnaris

"Mouth fungus" is a misleading old name. Cottony growth around the mouth is usually columnaris (a bacterium), not a fungus, which matters because antifungal products won't treat it.

Only a microscope confirms which it is. Merck notes columnaris bacteria appear as typical "haystack" clusters on a wet mount, which a vet can check quickly.

Popeye

What it is and why it happens Popeye (exophthalmia) means one or both eyes bulge out from the head. It's a sign rather than a disease. Causes include:

• One eye: usually injury, such as a bump against decor, a net, or jumping.
• Both eyes: more often infection (e.g. Aeromonas), poor water quality, internal disease, or gas bubble disease (supersaturated water).

Signs: a swollen, protruding eye; sometimes cloudiness or a white ring; reduced appetite. Severe cases can damage the eye permanently.

Look-alikes: normal large eyes in some bettas (compare both sides); cloudy eye from water quality without bulging; tumours behind the eye.

First steps: check water quality; look for sharp decor and other injury sources; Merck suggests examining the eye with a penlight. Many single-eye injury cases improve slowly with clean water. Infections may need vet-prescribed treatment.

Prevention: gentle netting (or use a cup), smooth decor, a lid, clean water.

When to see a vet: both eyes involved, rapid swelling, cloudiness, other signs of illness, or no improvement in about a week.

Dropsy

What it is and why it happens Dropsy is not one disease. It's the name for fluid build-up inside the body, which makes the belly swell and the scales stick out like a pinecone when seen from above. It usually signals serious internal illness: bacterial infection (such as Aeromonas), kidney or organ failure, or sometimes tumours or mycobacteriosis.

Signs

• Early: slight swelling, lethargy, poor appetite; scales starting to lift (check from above).
• Late: obvious "pinecone" scales, a very swollen body, bulging eyes, pale gills, stringy faeces, difficulty swimming.

Look-alikes: bloat or constipation (swollen, but scales lie flat), gravid female, tumour.

First steps and treatment approaches

• Move to a hospital tank with excellent water.
• Contact a vet. Antibiotics chosen after testing may help some bacterial causes.
• Sadly, by the time pineconing is obvious, the outlook is often poor. Your vet can help you decide when treatment is no longer kind, and can perform humane euthanasia.

Prevention: clean water, a good diet, quarantine, low stress.

When to see a vet: as soon as you notice scales lifting.

Mycobacteriosis (fish TB)

The Columbia research colony lists Mycobacterium among the most common betta diseases, and Merck describes it in aquarium fish generally. It's a slow, chronic bacterial infection with signs that vary: wasting, lethargy, skin ulcers, bent spine, popeye or loss of colour. There's no reliable cure, it can't be detected with non-lethal tests, and Merck notes it is zoonotic, meaning it can infect people through cuts (usually as skin infections). Wear gloves, keep tanks separate, and ask a vet about diagnosis, which usually needs post-mortem testing, and disinfection. Merck recommends 70% ethanol or phenolic disinfectants, as bleach alone isn't reliable against it.

Digestive and buoyancy problems

Constipation and bloat

What it is and why it happens Bettas are prone to digestive problems, mostly from overfeeding, dry foods that swell after eating, cold water, and lack of variety. A research colony also found unhatched brine shrimp eggs and shells caused gut blockages.

Signs

• Early: a rounder belly than usual, fewer or no faeces, less interest in food.
• Late: a very swollen belly, difficulty swimming or floating, lethargy.

Look-alikes: dropsy (scales lift: check from above), gravid female (rounded belly, vertical bars, visible white ovipositor), internal parasites, tumour, swim bladder disorder.

First steps

1. Fast for 24–48 hours. In a research colony, about half of bloated bettas recovered with a 24–48 hour fast.
2. Make sure the water is at the right temperature (76–81°F / 24–27°C).
3. Afterwards, feed small meals of soaked pellets, and try frozen daphnia.
4. Check for pinecone scales. If present, see Dropsy.

Tip: Epsom salt baths and peas are popular online remedies. There's little evidence for them in bettas, and peas aren't a natural food for a carnivore. Start with fasting, then ask a vet if it doesn't help.

Prevention: small meals, soaked dry foods, variety, a fast day, no brine shrimp shells. See Diet and feeding.

When to see a vet: bloating that lasts more than 2–3 days despite fasting, any pineconing, or a very swollen belly.

Swim bladder disorder

What it is and why it happens The swim bladder is a gas-filled organ that lets fish control buoyancy. "Swim bladder disorder" describes buoyancy problems: floating (positive buoyancy), sinking (negative buoyancy), listing to one side, or swimming tail-down. It's a sign with many possible causes, including overfeeding and constipation pressing on the bladder, poor water quality, infections, injury, tumours, and genetics or body shape (very round "balloon" or heavy-finned bettas may be more prone).

Signs: struggling to stay level, floating at the top unable to dive, lying on the bottom unable to rise, swimming on the side or upside down, or swimming vertically.

Look-alikes: exhaustion from strong current, heavy fins, very old age, severe illness, dropsy, constipation.

First steps

1. Test water and temperature.
2. Fast for 24–48 hours, then feed small, soaked meals.
3. Lower the water level temporarily and add a leaf hammock or plants near the surface, so your betta can rest and reach air easily.
4. Reduce the current.

What to expect: a research colony found that sinking in young fish (often linked to overfeeding) usually resolved as they grew, provided the fish could reach the surface. But floating fish often didn't recover, possibly because of organ damage. PetMD notes that vets can use X-rays to look for the cause.

Prevention: small meals, soaked foods, warm stable water, gentle flow.

When to see a vet: if buoyancy problems last more than a few days, or come with other signs.

Other conditions

Hole in the head

What it is: small pits or erosions on the head, sometimes along the lateral line (the sensory line along the side). It's often called "hole in the head" or head and lateral line erosion (HLLE). A 2021 review in the Journal of the American Veterinary Medical Association describes this kind of lesion as a response to suboptimal conditions, not a single disease. Suggested contributing factors include poor water quality, nutrition, stray electrical voltage, copper exposure, and, in studies of a marine fish, activated carbon in the filter. It's less common in bettas than in cichlids and marine fish. Keepers sometimes link it to the intestinal parasite Spironucleus/Hexamita, but that link isn't proven.

Signs: small white or pale pits on the head that can enlarge; later, larger eroded areas.

Look-alikes: injuries, columnaris, ulcers.

First steps: improve water quality; review diet; check heaters and equipment for stray voltage; consider whether long-term carbon use could be a factor. The JAVMA review recommends finding and fixing the underlying cause, plus supportive care and antimicrobials only when indicated.

When to see a vet: if pits spread, ulcerate, or appear with weight loss.

Tumours and growths

What they are: fish can develop tumours (benign or malignant) like other animals, as well as other lumps such as cysts, abscesses, granulomas (e.g. from mycobacteriosis), parasitic cysts, and lymphocystis, a viral infection causing cauliflower-like growths on skin or fins, which Merck notes is usually not life-threatening and often resolves without treatment. Merck's veterinary manual also describes vets surgically reducing some tumours in pet fish.

Signs: a lump on the body, fins, lips or gills; a one-sided swelling of the belly; a slowly changing body shape.

Look-alikes: constipation, dropsy, gravid female, popeye, ich (much smaller spots).

What to do: photograph the lump weekly to track growth. A vet can sample it to work out what it is and whether removal is possible. Many slow-growing lumps are compatible with a comfortable life; focus on quality of life.

When to see a vet: fast-growing lumps, lumps affecting eating, breathing or swimming, or any growth with ulceration.

Injuries and torn fins

What happens: fins tear on sharp decor, plastic plants, filter intakes, nets, or from fighting, nipping tank mates, or jumping. Long-finned males are particularly vulnerable. Scrapes and missing scales can occur too.

Signs: clean splits or holes in fins; missing scales; red marks; a bent fin ray.

Look-alikes: fin rot (ragged, discoloured, spreading), fin biting.

First steps

• Find and remove the cause (do the "tights test" on decor, check the filter intake, check tank mates).
• Keep water exceptionally clean to prevent infection; damaged tissue is where fungus and bacteria get in.
• Fins usually regrow over weeks. New growth is often clear or white at first, and can be slightly wavy.

Prevention: soft decor, a covered filter intake, a lid, gentle netting (or use a cup), no fin-nipping tank mates.

When to see a vet: deep wounds, wounds on the body, or signs of infection (white fuzz, spreading redness, rot).

Old age

What to expect: PetMD gives an average lifespan of about 3–5 years with good care. As bettas age you may see:

• More resting and sleeping, slower swimming
• Fading or changing colour (including more pale patches)
• Fins that are thinner, curl, or tear more easily
• A slightly curved back or thinner body
• Less interest in flaring and nest building
• More susceptibility to infection (the research colony noted fish over about 1.5 years seemed more prone to fin rot)

Caring for an older betta

• Lower the water level slightly and add resting spots near the surface.
• Gentle or very low flow.
• Smaller, softer meals; watch for weight loss.
• Skip flare training, or keep it very short and gentle.
• Keep the water especially clean and stable.

A gentle word about the end: when a betta can no longer eat, breathe comfortably, or move without struggling, and there is no treatable cause, talk to your vet about humane euthanasia. It is a final kindness. A vet can do it peacefully. Please avoid home methods you may read about online, as many cause suffering.

Sources

• https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/parasitic-diseases-of-fish
• https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/bacterial-diseases-of-fish
• https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/mycotic-diseases-of-fish
• https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/viral-diseases-of-fish
• https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/neoplasia-of-fish
• https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/management-of-aquarium-fish
• https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquatic-systems/environmental-diseases-of-aquatic-animals-in-aquatic-systems
• https://www.merckvetmanual.com/all-other-pets/fish/disorders-and-diseases-of-fish
• https://ask.ifas.ufl.edu/publication/FA033
• https://ask.ifas.ufl.edu/publication/FA006
• https://ask.ifas.ufl.edu/publication/FA084
• https://pmc.ncbi.nlm.nih.gov/articles/PMC9334006/
• https://www.petmd.com/fish/betta-fish-care-sheet
• https://www.petmd.com/fish/conditions/respiratory/swim-bladder-disorders-fish
• https://avmajournals.avma.org/view/journals/javma/259/6/javma.259.6.617.xml
• https://doi.org/10.1080/15222055.2011.635781
• https://fishvets.org/find-a-fish-vet/
• https://www.wavma.org/find-a-fish-vet