Fishes as well as other animals are subject to a wide spectrum of diseases. Fishes are affected by water temperature, pH, osmotic pressures, dissolved gases, elements, etc. These factors also determine whether or not a disease agent can or will cause disease in a fish.

Ich (ichthyophthiriasis) is one of the most prevalent diseases of fish. It is caused by a microscopic protozoan that causes white spots in the skin and gills of the fish. Alteration of the protective skin of the fish by ich allows opportunistic bacteria or fungi for invasion and the fish can die from secondary causes. The best control of ich is prevention. New plants and fish can introduce unsuspected ich.