Some disease symptoms for a pet fish are inflamed skin, drooping or chewed-on fins, a film of mucus over the body, and rapidly beating gills, according to Tetra. These are symptoms of the fish having an infestation of flukes, which are tiny flatworms.
Symptoms of gill flukes include skin and gills that look infected and inflamed, according to Tetra. If the fluke succeeds in destroying the fish's gills, the animal dies. Flukes are a sign of poor water quality, overcrowding, especially by species that are not compatible, and other stressors.
If a fish has clamped fin, it swims with its fins tucked against its body and appears lethargic, explains Tetra. Clamped fin is a symptom of several diseases and conditions, including poor water quality. A fish that has dropsy presents with protruding scales and bloating. Dropsy is caused by an infection of the fish's kidneys. Red pest is also a bacterial infection, according to Animal World. It shows up as streaks of blood and ulcers on the fish.
Fish can also contract lice, says Tetra. The symptoms of lice include agitation, rubbing against something to get rid of the parasite, and the sight of the lice on the fish. If the fish has a fungus, it appears as a cottony growth on the fish's body. Fungus kills the fish if it is not removed.