Fish health & disease
What the common illnesses look like, how to catch a sick fish early, and the established first-response and prevention principles — sourced to aquatic-health authorities, not aquarium folklore.
Educational, not veterinary advice. These guides explain what aquarists commonly see and the well-established principles of response and prevention. They do not prescribe medications or doses — for a seriously ill fish, or for any treatment specifics, follow the cited sources or consult an aquatic veterinarian.
Most fish illness is a husbandry story before it is a disease story: stable, clean water and a quarantine habit prevent far more losses than any medication cures. These guides stay deliberately practical — the signs to read, what they usually mean, and what to do first — and point you to the primary aquatic-health literature for the specifics.
- Diagnosis
Dropsy
The "pinecone" look isn't a disease — it's the visible end of failing fluid balance, and being honest about what it usually means is the first act of good care.
- Diagnosis
Pop-eye (exophthalmia)
A bulging eye is a sign, not a single disease — and whether one eye or both is affected is the first clue to which of several very different causes is at work.
- Diagnosis
Spotting a sick fish
Fish hide illness until it is well advanced. Recognising the early signs — and testing the water first — is the most useful health skill in the hobby.
- Diagnosis
Swim-bladder disorder
A fish that cannot hold its depth — floating, sinking, or swimming nose-down — has a buoyancy problem. The causes range from a temporarily overfed gut to a permanent deformity.
- Disease
Anchor worm & fish lice
Unlike most fish parasites, anchor worm and fish lice are large crustaceans you can see with the naked eye — and the wounds they leave are often more dangerous than the parasites themselves.
- Disease
Columnaris
Those cottony patches may not be fungus. Columnaris is a fast, gill-attacking bacterium — and treating it as a fungus, or raising the heat, costs fish.
- Disease
Fin rot
Ragged, receding fins are rarely the real problem — they're the visible end of a water-quality or stress failure, and a warning before it goes deeper.
- Disease
Fungus (Saprolegnia)
The cotton-wool tufts are real, but they are almost always the second problem — and the organism behind them is not even a true fungus.
- Disease
Gill & skin flukes
Microscopic hooked flatworms on skin and gills — flukes cause flashing and breathing trouble with no visible spot, and the two main kinds reproduce so differently that timing a treatment correctly depends on knowing which one you have.
- Disease
Hole-in-the-head (HITH / HLLE)
Pits and erosion on the head of a cichlid — a chronic condition tied to the Spironucleus flagellate, water quality, and nutrition simultaneously. No single fix works because no single cause explains it.
- Disease
Ich (white spot disease)
The white spots are the late sign of a fast, temperature-driven parasite — and only one stage of it can be killed, which is why timing decides the outcome.
- Disease
Velvet (gold-dust disease)
A fine golden dust and a flicking, gasping fish — velvet is ich's faster, gill-first cousin, and a parasite that can feed on sunlight.
- Husbandry
New-tank syndrome
The most common killer of beginner fish isn't a disease — it's the invisible chemistry of a tank whose filter hasn't caught up.
- Husbandry
Quarantine & hospital tanks
Two simple spare tanks prevent and contain more disease than any medication — and most aquarists skip them.