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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:

  • Increased mucus or slime on the body or gills
  • Darkened appearance
  • Hiding in an unusual or atypical location
  • Flashing (rubbing their bodies against a surface)
  • Rapid swimming and breathing or gasping at the surface
  • Slow movement (as the disease progresses)
  • Diminished appetite

Biology of Ich

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

Ciliated protozoans are:

  • Single-celled organisms with cilia, which are tiny hair-like structures used for movement and/or food gathering
  • The most common of all parasites affecting fish

    Common identifying features for freshwater ich include:

  • Cilia located over the entire surface
  • A U-shaped internal structure called the macronucleus
  • An overall gray, granular appearance
  • A slow, rolling movement
  • Commonly found within a cyst-like structure

    Ich has a complex life cycle, which takes 3-7 days to complete at 25 degrees C, but requires much more time at cooler temperatures.

Ich Life Cycle

  • Trophont (the feeding stage). This is the only stage seen by the naked eye – as the white spots on the fish. The mature trophont burrows under the skin and feeds on the skin and gills. It is well-protected from treatment chemicals.
  • Tomont (the reproductive stage). After infecting the fish, the trophonts fall off into the gravel and become encysted in a free-living dormant stage. Then they divide into hundreds of tomites.
  • Tomites (larval stage). Tomites are free-swimming larvae that can survive for about four days without a host, although they may be infective for only a portion of that time. It is this stage that is most vulnerable to chemical treatments in the water.
  • Theront (the infective stage). Tomites mature into theronts, which invade the fish by burrowing into the skin and gills and penetrate within five minutes. Here they feed on cells and tissue fluid until they mature into trophonts and begin the cycle all over again.


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:

  • Remove carbon filtration
  • Perform 50 percent or more water change prior to each new treatment
  • Monitor nitrite levels
  • Biological filtration should not be affected significantly, but if in doubt, monitor ammonia levels with test kit using salicylate reagent. Kits which use Nessler’s reagent will give a falsely elevated ammonia reading if formalin is present in the water

Other treatment options

  • Raising the temperature over 86 degrees F (30 degrees C) because of ich’s inability to complete its life cycle (for fish which can handle these temperatures)
  • Use of a prolonged salt bath over 1 ppt (such as 2 or 3 ppt)

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.


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