One eye or both eyes protrude from the head in an unusual fashion. There are various causes for this clearly visible disease.

In most cases, a deterioration of water quality is at the root of the problem. Checking pH, KH, CO₂ concentration, ammonium, nitrite and nitrate is urgently recommended. There have also been reports of vitamin deficiency in the diet leading to bulging eyes. Please check whether the fish have a varied diet and make sure that food containers are used up within a maximum of 3 months after they are opened. There are barely any vitamins left in the food 3 months after a food container has been opened. In that case, subsequent vitaminisation is then required.

Instead, the fish may have a viral infection (viral disease), with this especially applying to cold water fish like koi carps.

Bacterial diseases are a third possibility, particularly in warm water fish. The bacteria causing the disease stem from several groups. The only way to obtain a reliable diagnosis is to dye the bacteria.