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Ich (White Spot Disease)

Ichthyophthirius multifiliis, also known as “ich” or “white spot disease,” is the best known of all parasites affecting freshwater tropical fish. Ich can infect most species of freshwater fish and multiplies rapidly, causing devastating and rapid death within an aquarium.

Ich feeds on the skin and gills of fish, providing sites for infection by other organisms, such as bacteria and fungus. In addition, the resulting “holes” in the outer layers of the fish make it difficult for the fish to maintain the proper concentration of salts in the body.

Predisposing Factors

Any stress on a fish results in a reduced immune response and increased potential for infection. A few of these stresses include poor water quality (elevated ammonia or nitrite, sudden temperature changes, low dissolved oxygen), poor nutrition, crowding, improper social structure, and aggression. One of the most common causes of ich is introduction of a new fish into an established environment without adequate quarantine.


Ich appears as small, white, raised spots, similar to grains of salt, on the skin of your fish. These spots are actually the adult stage of the parasite known as the trophont, which is enveloped in the pus and tissue of the skin and slime layer. In addition, infected fish display one or more of the following symptoms:

Biology of Ich

Ich is one of a group of organisms known as ciliated protozoans.

Ciliated protozoans are:

Ich Life Cycle


Treat affected tropical fish with formalin and/or malachite green, every other day for three treatments, at 77 degrees F (25 degrees C). Because temperature affects the time required for development, increasing temperature will speed up the life cycle, and shorten total treatment time. At 59 degrees F (15 degrees C), each treatment should be applied every 3 or 4 days because the life cycle is prolonged.

Formalin or formalin/malachite green will work most efficiently if the fish are tolerant, especially when fish are heavily infected. However, formalin and malachite green are very toxic to fish if used improperly. Follow manufacturer’s directions. Some fish, such as pictus cats, some tetras and related fish, and elephant nose are very sensitive to the formalin/malachite green combination.

Although formalin can be used in ponds, formalin does remove oxygen from the water, so knowledge of oxygen levels is critical. Copper sulfate can be used in ponds, but alkalinity must be measured to determine potential toxicity and proper dosage rate. Copper is toxic to invertebrates and algae, and can adversely affect higher plants.

When treating entire tank or system with formalin and/or malachite green:

Other treatment options

Preventative Care

Quarantine new fish in a separate tank for a minimum of two weeks, but preferably three to four weeks, at 77 degrees F (25 degrees C) or higher. This will allow time for observation for disease and nutritional problems.