How to Check Your Fish for Bacteria and Fungus

See if the fish appears sluggish, hides, is not eating, has a swollen body (dropsy), red spots, red streaks or is bleeding on its body. These are all signs of an internal bacterial infection. One of the most common symptoms is dropsy, where the body bulges at the sides and stomach and the scales may even be forced to stick out. Bulging eyes, or "pop eye," is a common symptom of an internal bacterial infection.

Watch to see if the fish sinks to the bottom of the tank or floats at the very top of the water, but is not dead. This could be a swim bladder disorder commonly caused by bacteria. Head-standing, laying flat, floating upside-down or sideways, or poorly coordinated swimming are also signs.

Look for black patches on the fins and body along with body swelling. This is a bacterial infection called myxobacteriosis.

Watch for small black specks on the fish's body. Black spot, or diplopstomiasis, often occurs after the addition of new fish to the tank.

Suspect fin rot if the fins are whitish and ragged. Fungal and bacterial infections may also cause cottony white patches on the edges of the fins. Fungal fin rot often follows damage from an aggressive tankmate. Bacterial fin rot is similar, but not cottony. It's very contagious and can also be caused by poor water quality.

Look for a white or gray material covering just the eyes. This is an eye fungus.