Top 10 fish illnesses you may see - and what you can do about each one

Even well-kept tanks see illness eventually. A fish that was fine yesterday is sitting on the bottom today, or has a white fuzz growing on its side, or is swimming in circles. The difference between losing one fish and losing a tankful usually comes down to how quickly you identify what's wrong and how confidently you act. This guide covers the ten illnesses you're most likely to encounter, what they actually look like, what causes them, and what to do - including what most beginner guides skip.

1. Ich (White Spot Disease)

Ichthyophthirius multifiliis is the most common disease in freshwater aquariums. You'll see tiny white spots - each about the size of a grain of salt - scattered across the body and fins. Affected fish flash against decor, have clamped fins, and breathe faster than normal.

Ich is a parasite with a multi-stage life cycle. The spots you see are the feeding stage (trophonts) buried under the skin - they're actually unreachable by medication at that point. Treatment works by killing the free-swimming juvenile stage (theronts) in the water. This is why you must run a full treatment course of at least 10-14 days even if the spots disappear after a few days.

Raising temperature to 28-30°C (82-86°F) speeds up the parasite's life cycle, making the vulnerable free-swimming stage arrive sooner. This works well for most tropical fish, but don't do it with temperature-sensitive species like fancy goldfish or hillstream loaches. Combine heat with an ich-specific medication and keep doing water changes throughout.

2. Fin Rot

Fin rot shows as ragged, receding fin edges - sometimes with a white or red border along the degraded tissue. In bad cases the fin rays are exposed, and it can spread to the body. It's nearly always bacterial (Aeromonas or Pseudomonas species), though secondary fungal infection can occur on damaged tissue.

The real cause is almost always poor water quality or chronic stress - fin rot doesn't appear spontaneously in a clean, well-maintained tank. Check ammonia, nitrite, and nitrate before anything else. Ammonia and nitrite should be zero; nitrate should be below 20-40 ppm depending on your fish. If the numbers are off, do water changes first. Mild cases often heal with water quality alone. For persistent or spreading cases, use a broad-spectrum antibacterial treatment.

Also check for fin nippers. A betta being bullied by barbs, or a fancy guppy living with tiger barbs, will develop fin rot no matter how clean the water is.

3. Velvet

Oodinium pilularis (freshwater velvet) is often missed because the early signs are subtle. Rather than obvious white spots, you see a fine gold or rust-coloured dust - most visible when you shine a torch at a low angle across the fish's flanks. Fish flash, lose colour, breathe rapidly, and become lethargic. It spreads faster than ich and can kill a tank quickly if unaddressed.

Treatment follows a similar approach to ich: dim the lights (the parasite is photosynthetic in early life stages), raise temperature where safe, and use a velvet-specific medication - copper-based treatments are effective but must be dosed accurately and are lethal to invertebrates and some scaleless fish. Remove carbon from filters before treating.

4. Fungal Infections

True fungal infections - usually Saprolegnia or Achyla species - appear as white or grey fluffy patches that look like cotton wool on the skin, fins, or mouth. They're almost always secondary infections: the fungus colonises tissue that's already been damaged by injury, bacterial infection, or poor water quality. Healthy fish with intact slime coats rarely develop fungus unprompted.

Find and fix the underlying cause. If there's fin damage from aggression, separate the aggressor. If the water is poor, do water changes. Then treat with an antifungal medication. Salt (1-3 g/L) helps in early stages for salt-tolerant species but won't clear an established infection on its own.

5. Bacterial Infections (Body / Mouth)

Mouth rot (columnaris) and body ulcers are common bacterial presentations. Columnaris looks like a pale, eroding patch around the mouth that can spread quickly - it's often confused with fungus but has a more ragged, cheese-like texture rather than fluffy strands. Body ulcers appear as open sores, red patches, or areas of scale loss.

Both respond to antibacterial treatment, but speed matters. Columnaris in particular can kill fish within 24-48 hours in warm water. Improve water quality simultaneously - antibacterials work better in clean conditions.

6. Swim Bladder Disorder

A fish that floats at the surface, sinks to the bottom, or swims tilted and can't stay level has a swim bladder problem. The swim bladder is an internal gas-filled organ that controls buoyancy, and when it malfunctions the fish loses the ability to regulate its depth.

In most cases - especially with fancy goldfish and bettas - the cause is dietary: constipation or overfeeding causes internal pressure on the swim bladder. Fast the fish for 24-48 hours, then offer a small piece of cooked, de-shelled pea. This works surprisingly often. If it doesn't improve, the cause may be bacterial infection, a cyst, or a birth defect, in which case the prognosis is less clear.

7. Dropsy

Dropsy isn't a single disease - it's a symptom: severe fluid retention causing the body to bloat and the scales to lift outward in a distinctive pinecone pattern. It usually indicates organ failure, severe internal bacterial infection, or both. The kidneys can no longer regulate fluid, so it accumulates in the body cavity.

This is one of the hardest conditions to treat successfully. Isolate the fish immediately to reduce stress and prevent any possible spread. Some keepers try Epsom salt baths (1 tablespoon per 4 litres) to draw out fluid, and antibacterial treatment targeted at internal bacteria. But the honest reality is that by the time pineconing is visible, the internal damage is usually significant. Focus on prevention: consistent water quality, a varied diet, and not letting nitrates creep above 40 ppm.

8. Pop-eye (Exophthalmia)

One or both eyes protrude abnormally from the socket - not just slightly, but visibly bulging. Unilateral pop-eye (one eye) is usually caused by localised injury or bacterial infection. Bilateral pop-eye (both eyes) more often signals a systemic internal infection or poor water conditions.

The eye itself is unlikely to fully recover even if the infection is cleared, but the fish can survive and live well with reduced or lost vision in that eye. Treat with antibacterial medication, improve water quality, and keep the fish in a low-stress environment. Don't assume injury automatically - check your water parameters first.

9. Hole-in-the-Head Disease (HITH)

Common in cichlids - particularly oscars, discus, and other large South American species - HITH appears as small pits or eroding lesions on the head and along the lateral line. It's associated with the protozoan Hexamita, but elevated nitrates, activated carbon use, and poor diet all appear to contribute. The hobby debates the exact cause, and honestly the evidence suggests it's multi-factorial.

Treatment with metronidazole (targeting Hexamita) often helps, but cleaning up the tank environment is just as important. Cut nitrates with more frequent water changes, remove activated carbon from the filter, and improve diet with varied, nutritious foods including fresh vegetables for herbivorous cichlids.

10. Anchor Worm and Fish Lice

Lernaea (anchor worm) and Argulus (fish lice) are macroscopic external parasites - you can actually see them with the naked eye. Anchor worms appear as thread-like protrusions from the skin, often with redness around the attachment point. Fish lice are flat, disc-shaped parasites up to 10 mm across, crawling on the fish's surface.

Both cause irritation, secondary infection at attachment sites, and significant stress. Treatment involves anti-parasitic medications (organophosphate-based treatments like trichlorfon are traditional, though availability varies by country). Individual parasites can be manually removed with fine forceps after the fish has been sedated or carefully netted - but treat the whole tank, as juvenile stages are in the water.

Prevention: What Actually Works

The conditions that make fish get sick are almost always the same: a tank that isn't properly cycled, overcrowded stocking, inconsistent water changes, and skipping quarantine for new arrivals. Get those four things right and you'll rarely need this guide.

Check your water parameters regularly - ammonia 0, nitrite 0, pH stable for your species, temperature consistent. Know how often to do water changes for your stocking level and stick to it. And if you're still choosing fish, start with species that are genuinely hardy - they give you more margin for error while you find your feet.