Costia

Symptoms

Spotting illness early gives the best chance of a cure, so check your fish daily at feeding time. A grey-blue cloudy film of excess slime on the skin, flashing, clamped fins, lethargy and laboured breathing. Spots are not visible to the eye. If you see several of these signs together, act promptly rather than waiting.

Causes

A tiny protozoan parasite that attacks the skin and gills of stressed fish, multiplying fast in poor water. In nearly every case the underlying trigger is stress, poor water quality, or the introduction of an infected fish - all of which weaken a fish's natural defences.

Treatment

Raise temperature where the species allows, improve water quality, and treat with a proprietary anti-parasite medication (formalin/malachite-based). Treat the whole tank promptly.

While treating, increase aeration, remove activated carbon from the filter (it absorbs medication), finish the full course even after the fish looks better, and keep the water immaculate with regular changes. Always dose for the tank's true water volume and follow the product instructions.

Prevention

Quarantine new fish and keep water clean and stable, as this parasite mainly strikes stressed fish. The golden rules are simple: quarantine every new fish for 2-4 weeks, never overstock or overfeed, avoid sudden temperature or chemistry swings, and do regular partial water changes. A stable, clean, low-stress aquarium prevents the large majority of disease.

FAQ

This information is general guidance, not veterinary advice. Consult an aquatic vet for serious cases.