Ichthyophthirius multifiliis is a species of ciliate protozoan which parasitizes freshwater fish; the disease it causes is usually called ich or white spot. It is probably the most common aquarium fish parasite and there are few Aquarists that have not met it on one or more occasions. "Ich" (the colloquially abbreviated form of the name) is the largest known parasitic protozoan found on fishes. Adult organisms are oval or round and measure 0.5 to 1.0 mm in size. The adult is uniformly ciliated and contains a horseshoe-shaped nucleus which can be seen in older individuals. The disease becomes especially serious in enclosed areas, where it spreads quickly from one fish to another. Ich is the disease responsible for the most fatalities in freshwater aquarium fish and can cause notable damage to aquaculture. Marine ich is caused by a different ciliate, Cryptocaryon.
After approximately one week of parasitism, mature trophozoites leave their host, settle to a substrate and secrete a cyst. The encysted cell, called a tomont, undergoes rapid division over approximately twenty-four hours to produce 600-1000 daughter cells called tomites. Once these reach maturity, they exit the cyst and develop into a theront stage, which is highly mobile. Theronts then infect new fish, digging their way into exposed parts, under the scales, or more commonly into its gill plate. The entire life-cycle takes about seven to ten days to complete.
Adding a small amount of aquarium salt according to the directions on the aquarium salt package may also help prevent Ich. However, beware that not all freshwater fish can handle salt in their water. Some "skin fish," like most loaches and some catfish, are very susceptible to burning from salt.
Ich outbreaks usually occur when new fish are introduced or during times of stress. Elevated ammonia or nitrate levels or sudden changes in temperature can also bring about a latent infection. A sudden chilling of the fish, which can easily occur when they are being transported, is often sufficient to take the parasite from its latent state to the reproductive phase.
Typical behaviours of ich infected fish include:
Tropical fish are vulnerable to a large variety of diseases. As such, ich treatment should not be applied unless one or more of the fish is exhibiting the characteristic white spots.
Ich infections are usually visible in the form of characteristic white spots on the side of the fish. The white spots are pockets of fish epithelia containing Ichthyophthirius cells called trophozoites or trophonts, which feed on the tissues of the host and may grow to 1 mm in diameter. A smear should show ciliates if white spot is present.
Fins are folded and show white spots about 1 mm in diameter.
Eyes may appear cloudy or milky.
Any treatment method must take into account the species of fish (some will not tolerate certain medications), how high the infection rate is, and the size and type of environment.
If it is detected before it becomes too serious, a number of different treatments can be applied. Only the free-swimming stage of the parasite is susceptible to treatment; neither the trophonts under the epithelium nor the tomont cysts can be killed.
Heat treatment can be highly effective, and it can be combined with other treatments.
The three phases of the ichthyophthirius life-cycle (Adult, Cyst, Free swimming) take about 4 weeks at 21 °C (70 °F) to complete but only 5 days at 27 °C (80 °F). For this reason it is recommended that the aquarium water be raised to 28-30 °C (82-86 °F) for the duration of the treatment. Avoid fast temperature changes; water temperature should be raised or reduced gradually 0.5-1 °C (1-2 °F) per day. There are species of fish that will not tolerate the high end of temperatures needed to be effective. If the fish can stand it, raise the temperature even higher, up to 30 °C (86 °F). Raising the temperature also, presumably, reduces and kills the free swimming parasite. It is important to remember that raising the temperature higher, but not high enough so that the parasite is killed, should be used in conjunction with some sort of medication. The heat speeds up the life cycle of the ichthyophthirius, which is useful if the fish is being medicated, because otherwise the parasites simply reproduce at a faster rate, and kill the aforementioned fish quicker. Temperatures at or above 30 °C (86 °F) are generally considered to be fatal to ich
Salt treatment can be combined with heat treatment.
Vacuuming the surface of the gravel with a siphon device can remove the parasite at cyst stage. Try not to stir up the water too much as it just blows the cysts all over.
All medications, to some degree, are toxic not only to the parasite but also to the fish. Grossly weakened fish will not tolerate medication that more robust and less infected ones may. Bottom feeders such as catfishes, scaleless fish, and many tetras are adversely impacted by the use of malachite green.
Malachite green is hazardous to handle: it is known to cause blator problems, mutations, and is harmful to fetuses. Gloves and a protective mask should always be worn when handling the concentrated powder. Pregnant women should never handle this chemical. There are claims that malachite green might increase in toxicity to fish as the temperature increases. You may want to reconsider your decision to use malachite green if you intend to raise the temperature at the same time, or if you already maintain your temperature at a higher level than normal. Malachite green also tends to stain the plastic and silicone in the aquarium.
If the disease is diagnosed early and effectual treatment is applied, the outlook is excellent. However if the infestation is at an advanced stage, mortalities should be expected. Note that when a fish has been cured from Ich, it will usually form an immunity to the infection.