Common fish diseases

What you're seeing

Tiny white spots (about 1mm across) covering the body and fins, typically appearing first as just one or two before spreading rapidly across the fish. The fish may flash (scrape against decor) and show increased gill movement.

What it is — ich (white spot disease)

Ichthyophthirius multifiliis is a parasite with a temperature-dependent life cycle. The visible white spots are encysted parasites in fish tissue, invulnerable to medication. The vulnerable stage is the free-swimming "theront" released when spots burst — this is what treatment must target across 10-14 days.

Treatment approach

Raise temperature to 28-30°C for species that tolerate it (this accelerates the parasite life cycle, shortening treatment time). Use a commercial ich medication following the product's specific instructions for the full course — do not stop when spots disappear, continue for several days after. Many keepers add aquarium salt at low doses alongside medication; salt is genuinely effective for ich on species that tolerate it, but is harmful to many catfish and shrimp.

Common mistake: Stopping treatment when spots disappear. The parasite has simply entered the encysted stage; without continued treatment, it returns within days.

What you're seeing

Fin damage starts as a slightly ragged or discoloured edge, often with a faint white or grey margin. Progresses to obvious fin erosion, with chunks missing or fin tissue rotting back toward the body. The fish may also show clamped fins or reduced activity.

If the damage appeared suddenly rather than gradually — particularly as crescent-shaped bites or large torn pieces — this is fin nipping by tankmates, not disease. Look at your stocking; common culprits are tiger barbs, serpae tetras, and overcrowded danios attacking long-finned species. The fix is rehoming the aggressor, not medication.

What it is — fin rot

A bacterial infection caused by opportunistic bacteria (often Aeromonas, Pseudomonas, or Vibrio species) that attack stressed or injured fish. Fin rot is functionally a water-quality disease, not an infectious one — the bacteria are present in every tank but only attack compromised fish.

Treatment approach

Address the underlying water issue first — large water changes, restore parameters, identify and remove any aggression source. For mild cases (small amount of fin damage), water quality alone often resolves the problem within 1-2 weeks. For advanced cases (significant erosion, body damage), antibacterial medication is appropriate alongside water improvement.

Common mistake: Medicating without fixing water quality. The bacteria will return as soon as treatment stops if the underlying environmental issue isn't addressed.

What you're seeing

Distinctive fuzzy white growth that looks like cotton wool or fine mould, typically on the body, fins, or around the mouth. Often appears after physical injury or alongside other stressors.

If the white patches are flatter and more skin-like rather than fluffy, particularly around the mouth or in a saddle-shape across the body, this is likely columnaris (see next section) rather than fungus. The distinction matters because columnaris is bacterial and progresses much faster.

What it is — fungal infection

True fungal infection (usually Saprolegnia or related species) typically appears at sites of injury, dead tissue, or unfertilised eggs. Healthy fish are resistant — fungus is an opportunistic infection of already-compromised tissue.

Treatment approach

Improve water quality. Treat the underlying injury or stress that allowed the fungus to take hold. Commercial antifungal treatments (such as methylene blue baths or commercial antifungal products) work well for localised infections. Salt baths help on species that tolerate salt.

Common mistake: Mistaking columnaris for fungus and treating with antifungal medication, which doesn't address the bacterial cause. The fish often dies during the delay.

What you're seeing

Flat white or grey patches that look skin-like rather than fuzzy, often around the mouth (sometimes called "mouth fungus" despite being bacterial), in a saddle shape across the back, or on fins. Fish may also show rapid breathing and lethargy.

What it is — columnaris

A bacterial infection (Flavobacterium columnare) that's highly contagious and can kill fish within 24-48 hours in acute cases. Significantly more dangerous than the fungal infection it's often mistaken for.

Treatment approach

Urgent. Improve water quality immediately (large water change). Lower temperature slightly if possible — columnaris is more aggressive at higher temperatures. Treat with an antibacterial medication effective against gram-negative bacteria. Kanamycin and certain other antibiotics are commonly used internationally, but availability varies in Australia (see Australian medication section).

Common mistake: Treating as fungus, losing critical days while the bacterial infection progresses.

What you're seeing

Fine golden or rust-coloured dust covering the body, often more visible under direct light at an angle. Fish may show flashing, rapid breathing, and lethargy.

What it is — velvet (oodinium)

A parasite (Piscinoodinium or Amyloodinium) significantly more dangerous than ich. Velvet attacks gills before becoming visible on the body, so by the time you see the golden dust, the fish may already have severe gill damage. Highly contagious and can wipe out a tank quickly.

Treatment approach

Urgent. Velvet parasites use photosynthesis, so darkening the tank significantly impairs their reproduction — black out the tank entirely for the duration of treatment. Raise temperature slightly. Treat with a copper-based medication (the standard velvet treatment), following product instructions exactly.

Common mistake: Not removing invertebrates before copper treatment, killing the shrimp and snails alongside the parasites.

What you're seeing

The fish's body becomes swollen and, crucially, the scales begin protruding outward, giving a pinecone-like appearance when viewed from above. The fish may also become lethargic, lose colour, and stop eating.

If the fish is bloated but the scales aren't sticking out, the problem is usually digestive rather than systemic — see the swim bladder / constipation section below.

What it is — dropsy

Not a single disease but a symptom of severe internal organ failure (usually kidney), causing fluid retention that bloats the body and protrudes the scales. Underlying causes include bacterial infection (often Aeromonas), viral infection, or organ failure from prolonged exposure to poor conditions.

Treatment approach

Often unsuccessful. By the time dropsy is visible, organ damage is usually irreversible. Supportive care includes isolating the fish in a hospital tank, maintaining excellent water quality, treating with a broad-spectrum antibacterial medication, and in some cases adding Epsom salt to help draw out fluid. Many keepers consider visible dropsy a humane-euthanasia situation rather than a treatable disease.

Common mistake: Aggressive medication of an already-failing fish, prolonging suffering with minimal hope of recovery.

What you're seeing

The fish has obvious difficulty maintaining its position in the water — floating upside down, sinking to the bottom, or swimming in tight circles unable to right itself.

What it is — swim bladder disorder

A disorder of the gas-filled organ that controls buoyancy. Causes range from mild (overfeeding, constipation) to serious (bacterial infection, genetic deformity). Particularly common in heavily-bred species like fancy goldfish and balloon mollies due to deformed body shapes.

Treatment approach

For mild cases caused by overfeeding or constipation, fast the fish for 2-3 days, then feed a deshelled boiled pea. For persistent cases, look for underlying infection — raise temperature slightly and consider antibacterial medication if needed. Genetic swim bladder issues in heavily-bred species are usually permanent and can only be managed, not cured.

What you're seeing

Fish positioning themselves at the surface with rapid gill movement. Note: bettas, gouramis, and some catfish naturally surface to breathe atmospheric air — this is normal behaviour for these species, not a symptom.

What it likely is — oxygen deprivation

For non-labyrinth species, surface gasping indicates oxygen deprivation. Possible causes:

• Low dissolved oxygen — overcrowded tank, high temperature, poor surface agitation, or plants consuming oxygen overnight
• Ammonia or nitrite damage to gills — water testing confirms; the fish can't process oxygen through damaged gills
• Gill flukes or velvet — gill-stage parasites prevent oxygen absorption

Treatment approach

Test water immediately. If parameters are off, do a large water change. If parameters are fine, add an air stone or increase surface flow. If only some species are gasping and others are fine, look for species-specific gill damage or parasite infection.

What you're seeing

Stringy white faeces (rather than normal coloured pellets), gradual weight loss despite normal or increased appetite, persistent bloating, and gradual decline. Often arrive with new fish, particularly wild-caught specimens.

What it is — internal parasites

Various worms and protozoa that infect fish digestive systems. Identification ideally requires microscopic examination of faeces, which most home keepers can't do — treatment usually proceeds on suspicion rather than confirmed diagnosis.

Treatment approach

Treat with a broad-spectrum dewormer — praziquantel for most worms, metronidazole for protozoa. Quarantine for treatment because the medication affects the whole tank ecosystem. Available in Australia under various brand names sold for koi and tropical fish.

What you're seeing

Loss of appetite — fish ignores food, spits it out, or seems uninterested at feeding time.

What it likely is

Loss of appetite is one of the earliest signs of illness but also has many non-disease causes. Work through these in order:

1. Water quality. Bad parameters suppress appetite before visible symptoms appear.
2. Stress from tankmates or environment changes. Recently added fish often stop eating for a few days while acclimating.
3. Wrong food. Some species genuinely refuse certain food types (scarlet badis won't eat flake; certain catfish need sinking pellets).
4. Temperature. Cold fish don't eat. Check your heater and thermometer.
5. Disease. Once the above are ruled out, look for other emerging symptoms.

Treatment approach

Address the cause based on what you find. A healthy fish can go a week without food without harm — don't compensate by feeding more, as uneaten food pollutes water and worsens any underlying issue.

What you're seeing

A fish that previously swam openly is now hiding constantly, refusing to come out even at feeding time, or hovering near the bottom or behind decor.

What it likely is

Behavioural changes can signal illness or environmental stress. Common causes:

• New fish are often shy for days or weeks. Normal acclimation.
• Aggressive tankmates. Look for fin damage or dominant behaviour from other fish.
• Bright lighting or open tanks. Many species need cover (floating plants, dense planting, caves) to feel secure.
• Wrong group size. Schooling species kept in groups smaller than 6 become withdrawn.
• Early illness. Sick fish often retreat before showing visible symptoms.

Treatment approach

Address whatever environmental cause matches your situation. If no environmental cause is apparent, watch carefully for emerging symptoms over the next few days.