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Aquarium fish diseases - start here
Last updated: May 2026
- Most diseases are triggered by stress: poor water quality, overcrowding, bad diet, or sudden temperature swings cause the immune system to fail before pathogens take hold.
- Ich, fin rot, and velvet are the three most common diseases beginners encounter - all are treatable if caught early.
- New tank syndrome kills more fish than disease: ammonia and nitrite spikes from an uncycled tank look like illness but aren't - fix the water first.
- Quarantine before you treat: move sick fish to a separate tank before adding medication. Treating the main tank nukes your beneficial bacteria.
- Dropsy and swim bladder problems are symptoms, not diseases - the underlying cause determines whether treatment is possible.
- Check parameters first, always: a free ammonia test costs less than a bottle of medication and will often explain everything.
Why fish get sick - the short version
Fish don't randomly catch diseases the way humans do. In almost every case, something in the environment broke down first. Pathogens like Ichthyophthirius multifiliis (the ich parasite) are present in most aquariums all the time, held in check by a fish's immune system. Stress suppresses that immune system. Bad water quality, wrong temperature, wrong tank mates, or a poor diet are the usual suspects.
This matters because it changes how you approach treatment. Pouring medication into a tank with high ammonia won't save a fish - the water chemistry will finish the job before the drug has a chance. Fix the environment first, then treat the disease.
The aquarium trade sells a lot of medication. Most of it gets used on fish that were actually dying from ammonia poisoning or temperature shock. Before you buy anything, test your water. Ammonia above 0.5 ppm or nitrite above 0.25 ppm is more likely to be killing your fish than any parasite.
The six conditions this guide covers
Each condition below has its own dedicated page. This hub gives you enough to identify what you're dealing with and act quickly.
Ich (white spot)
The most common freshwater disease in the hobby. You'll see small white grains - like salt crystals - scattered across fins and body. Fish scratch against objects and may clamp their fins. Ich is caused by a protozoan parasite with a life cycle that includes a free-swimming stage, which is the only stage medication can kill.
Treatment: raise temperature to 28-30°C (82-86°F) to accelerate the parasite's life cycle, then treat with a copper-based or formalin-based medication. The whole cycle takes 7-10 days at higher temperatures. Don't stop treatment early - you'll miss the next wave of free-swimmers.
Stopping ich treatment after three days because the spots disappear. Those spots are the adult stage - they drop off to reproduce. The free-swimming offspring are still in the water. Continue treatment for the full recommended course or the outbreak will return within a week, often worse than before.
Fin rot
Fin edges become ragged, discoloured (white, brown, or black), and recede toward the body. In mild cases it looks cosmetic. Left untreated, it progresses to the body and becomes life-threatening. Fin rot is usually bacterial (Aeromonas or Pseudomonas species), though fungal fin rot also occurs and looks slightly different - white fluffy edges rather than jagged tissue loss.
The cause is almost always water quality. Fix your nitrates, increase water change frequency, and mild cases often heal on their own. Severe cases need antibacterial treatment - in the UK and Europe this usually means aquarium antibacterials available over the counter; in the US, products like Maracyn are accessible. Salt (1 tablespoon per 10 litres) as a short-term bath can help early-stage bacterial fin rot.
Velvet (Oodinium)
Often mistaken for ich at first glance, but the spots are finer - more like a gold or rust-coloured dust coating the fish, especially visible on dark-coloured fish under a torch held at an angle. Fish flash, clamp fins, and breathe rapidly. Velvet is more aggressive than ich and can kill fish faster.
Treatment requires darkness (cover the tank - the parasite is photosynthetic in its early life stage) and medication. Copper sulphate treatments are most effective. Raise temperature slightly to accelerate the life cycle. Velvet is highly contagious; quarantine any new fish for two weeks before adding them to an established tank.
Swim bladder problems
A fish that floats at the surface, sinks to the bottom, or swims at a permanent angle is showing swim bladder dysfunction. The swim bladder is an internal gas-filled organ that controls buoyancy. When it's not working, the fish can't maintain its position in the water.
Here's the complication: swim bladder dysfunction is a symptom, not a single disease. Causes include constipation (very common in fancy goldfish and bettas), bacterial infection, physical injury, parasites, or - in long-term cases - cysts and tumours. A fish that's floating after two days of fasting and a pea-based laxative diet probably just had a digestive issue. A fish that's been listing for three weeks despite treatment likely has something internal that won't resolve.
Before assuming disease, fast a swim bladder fish for 48-72 hours, then feed a small amount of blanched, skinned pea. Constipation is the most common cause in fancy goldfish and bettas, and this simple approach resolves it more often than you'd expect.
Dropsy
Dropsy is one of the most misunderstood conditions in freshwater fishkeeping. The fish's abdomen swells dramatically, and the scales lift outward - the classic "pinecone" appearance when viewed from above. It looks striking and alarming, which is why it gets a lot of attention.
The honest reality: by the time a fish shows full dropsy symptoms, the internal organs (usually the kidneys) are already severely damaged. The swelling is fluid accumulation caused by organ failure, not the disease itself. Bacterial infection (Aeromonas spp.) is usually involved, but it's often secondary to an immune system compromised by long-term poor conditions.
Treatment is rarely successful once symptoms are pronounced. Antibiotic treatment in a quarantine tank with pristine water is worth attempting, particularly for fish caught very early. But euthanasia is often the more humane choice for a fish with full-blown dropsy. The dedicated dropsy page covers how to make that call honestly.
Most dropsy guides end with "treat with antibiotics and hope for the best." That's not wrong, but it undersells how poor the prognosis usually is. A fish with raised scales and a fully distended belly has maybe a 10-20% survival rate even with ideal treatment. Prevention - good water quality, no overcrowding, varied diet - is the only reliable strategy.
New tank syndrome
Strictly speaking, new tank syndrome isn't a disease - it's ammonia and nitrite poisoning from an uncycled aquarium. But it kills far more fish than any pathogen, and the symptoms (lethargy, gasping at the surface, red-streaked fins, sudden death) look identical to disease.
A new tank without an established nitrogen cycle has no beneficial bacteria to process fish waste. Ammonia builds up, then nitrite. Both are toxic at very low concentrations - ammonia above 1 ppm is dangerous; nitrite above 0.5 ppm causes damage to gill tissue and red blood cells. Fish that survive the acute phase often develop secondary infections because their immune systems have been wrecked.
The fix is cycling the tank before adding fish. If you already have fish in an uncycled tank, large daily water changes (30-50%) are the only way to keep ammonia and nitrite from reaching lethal levels while the cycle establishes. Read the full guide on how to cycle a fish tank before you stock anything new.
A fish-in cycle takes 4-6 weeks on average - during which ammonia and nitrite must be tested and managed daily to keep fish alive.
The triage process - what to do when a fish looks sick
Work through this sequence before reaching for medication:
1. Test the water. Ammonia, nitrite, nitrate, pH, temperature. Check your readings against the baseline ranges in the aquarium water parameters guide. A single bad reading tells you more than any symptom description.
2. Identify the symptom pattern. White spots or gold dust (parasites), ragged fins (fin rot), bloating (internal), listing or floating (swim bladder), and pineconing (dropsy) all point in different directions. Don't treat blind.
3. Quarantine if possible. A cheap spare tank or even a large plastic tub with a heater and sponge filter keeps a sick fish away from healthy tank mates and lets you treat without killing your biological filter.
4. Treat the right thing. Bacterial infections need antibacterials. Parasites need antiparasitic treatments. Fungal infections need antifungals. Using the wrong medication achieves nothing and may make things worse by stressing the fish further.
5. Review husbandry. Once the immediate crisis is managed, figure out why it happened. Check stocking density against your tank size using the free stocking calculator, review your water change routine, and look at your feeding amounts. Most disease outbreaks are preventable.
Medication cocktails are dangerous. Mixing treatments - say, an antibacterial and a parasite treatment simultaneously - can be toxic to fish and will almost always crash your beneficial bacteria. Treat one thing at a time unless a product is specifically formulated as a combined treatment.
Prevention is the only reliable strategy
This sounds obvious, but it's worth being direct about what prevention actually means in practice:
Quarantine all new fish for two to four weeks before adding them to an established tank. Ich, velvet, and bacterial infections all hide in fish that look perfectly healthy. A quarantine tank doesn't need to be elaborate - a 40-litre tub with a heater, sponge filter, and a place to hide is sufficient.
Don't overstock. Overstocking drives up ammonia and nitrate faster than any filter can handle, suppresses immunity through chronic stress, and increases the chance of aggression and injury - an open wound is an infection waiting to happen. If you're not sure whether your stocking makes sense, the stocking calculator is a good sanity check.
Feed varied, appropriate diets. A betta fed exclusively on flake food for two years is nutritionally deficient. Varied diet (live or frozen daphnia, bloodworm, appropriately sized pellets) keeps immune systems functional.
Maintain the nitrogen cycle. Regular partial water changes, avoiding sudden filter replacements, and not over-cleaning the substrate are all part of keeping the beneficial bacteria population healthy. The nitrogen cycle guide covers this in full.
Track your parameters and get care reminders with App-aquatic.
Get the free appCan I treat ich without raising the temperature?
Yes, medication alone can work, but it takes longer and requires more precise dosing because the parasite's life cycle runs slower at lower temperatures. Raising temperature to 28-30°C (82-86°F) alongside medication is faster and more reliable. Don't raise temperature if you're keeping fish that can't tolerate it - goldfish, for example, shouldn't be pushed above 24°C (75°F).
My fish has white spots but I'm not sure it's ich - what else could it be?
Velvet can look similar but the spots are finer and more gold or rust-coloured - check with a torch in a dark room. Lymphocystis produces larger, cauliflower-like white lumps rather than even spots, and is viral so medication won't help. Fungal infections are usually fluffy or cottony rather than distinct spots. If it's uniform, small, and scattered across the fins and body, ich is the most likely diagnosis.
Do I need to medicate the whole tank or just the sick fish?
For parasitic diseases like ich and velvet, yes - treat the whole tank, because free-swimming parasites are already in the water and will infect other fish. For bacterial fin rot in a single fish, a quarantine tank treatment is preferable so you don't stress healthy fish or damage your biological filter. When in doubt, quarantine and treat separately.
How do I know if my fish is dying from disease or from water quality?
Test your water before you do anything else. Ammonia or nitrite above safe levels, or a pH crash, will cause gasping, lethargy, and rapid death that looks exactly like disease. If parameters are bad, a large water change is the first intervention - not medication. If parameters are fine and symptoms persist, then investigate disease causes.
Is there any point treating a fish with dropsy?
Sometimes, if caught very early - meaning mild swelling and no raised scales yet. Antibiotic treatment in a clean quarantine tank with added aquarium salt (1-3 g per litre) is worth trying. Once the scales are fully raised and the fish is visibly distressed, survival is unlikely regardless of treatment. Euthanasia with clove oil is a humane option and worth knowing how to do.