It is bacterial and almost always triggered by poor water quality or stress. Fix the water first — frequent changes and clean parameters — then treat with an antibacterial.

Common aquarium fish diseases

Updated 2026-06-146 min read

Most aquarium fish diseases take hold when fish are already stressed by poor water, a chill, or a rough move home. Catching them early — and fixing the underlying water quality — gives the best chance of recovery.

This guide covers the diseases you are most likely to meet, the symptoms that identify each one, and the first-line treatments. When in doubt, test your water and quarantine the affected fish.

Steps

Ich (white spot)
A parasite that shows as tiny white grains, like salt, scattered over the body and fins. Treat by gradually raising temperature and using an ich medication, dosing the whole tank.
Ich often follows a temperature drop or new-fish stress. Stable warmth and quarantine are your best prevention.
Fin rot
A bacterial infection that frays and erodes the fins, often with a white or reddened edge. It is usually a symptom of poor water — improve water quality and use an antibacterial treatment.
Fungus
Cottony white or grey tufts on the body, mouth, or an injury. Fungus targets damaged or weakened fish; treat with an antifungal medication and remove any decaying matter from the tank.
Dropsy
A swollen belly with scales sticking out like a pinecone — usually a sign of internal bacterial infection and organ failure. It is hard to cure; isolate the fish early and treat with antibacterials.
Velvet
A parasite giving a fine gold or rust-coloured dust over the skin, with clamped fins and rapid breathing. Treat in dim light with a copper or anti-parasitic medication, dosing the whole tank.
Fix the root cause
Disease usually rides in on bad water or stress. Test your parameters, do a water change, and ask the AI assistant to weigh your symptoms and readings together.
Aquairi AIWhy is my pH dropping?Low KH lets pH swing — test carbonate hardness.