White Spot Syndrome / ich (Ichthyophthirius multifiliis) is perhaps the most common disease encountered by freshwater aquarists everywhere, both beginning and advanced. It is caused by a protozoan parasite with a complex life cycle – in order to understand how to treat it successfully, you will need to understand a few things about this life cycle.
Low levels of Ich usually result in fish that scratch against inanimate objects in the tank, such as gravel and ornaments. Research has shown that fish can develop partial immunity to low levels of pathogens in their water and establish a tentative balance. Scratching itself should not be cause for real alarm, because some fish can live for years in this state if otherwise kept in optimal conditions, but they should be closely observed just in case. Sudden changes in their environment, such as temperature/pH fluctuations, bad water quality, overcrowding, etc. can disrupt this delicate equilibrium and spur on serious outbreaks of disease (these conditions cause stress, which suppresses the fish’s natural immunity). Of special note here are tanks that have not completed cycling (only just set up, but which don’t have established nitrifying bacteria in the filter yet), tanks that were overstocked almost overnight, and cases of sudden changes in ambient room temperature. These are especially common in the fall and winter, when ambient temperatures can change so fast that the heater’s thermostat lags behind and gives your fish “the chills”. Note that, in all these scenarios, Ich should be understood as a consequence of poor fish keeping conditions, and there´s no point in worrying about treating this consequence without first attacking and correcting the cause of the poor conditions in which the fish are living.
The main symptom of this disease is the small sugar-grain like spots which give the disease one of its common names. Fish may also rub against hard objects. In advanced stages, fish may be observed gasping and gill damage may be apparent.
The disease is caused by a protozoan parasite, Ichthyophthirius multifiliis, and is very contagious. The parasites spend a portion of their life cycle embedded beneath the skin of the fish, where they feed from the body tissue. The white cyst in which they are enclosed gives the characteristic white spots. Adults emerge from these cysts and fall to the floor of the aquarium, where they multiply inside a protective capsule. Some time later, the capsule bursts, releasing hundreds of free-swimming infectious parasites which attach themselves to new hosts. White spot is often introduced with new fish which have not been quarantined. Outbreaks of white spot often occur after fish have been exposed to cooler than normal temperatures. Clown loaches are particularly prone to white spot.
There are two main approaches to curing whitespot, and opinions vary on which is the more effective. There are several effective commercially available remedies, normally based on malachite green and formalin. Note that malachite green is hard on scaleless fish like catfish and loaches, and also other fish such as tetras. Some alternative medications are based on copper and formalin. The other method employed is to add salt to the tank (gradually), up to a level of 6-8 tsp per gallon. Note that fish vary in their tolerance of salt, and for more sensitive soft-water species, it may be better to use 3-4 tsp per gallon maximum. Higher salt levels may also affect plant growth.
In either case, increasing the temperature should kill the parasite off more quickly, because it will speed up the life cycle of the parasite, so that the free-swimming stage is reached as quickly as possible – this is the only stage affected by medications. However, increasing the temperature means there will be less oxygen dissolved in the water (some medications can lower it too), so ensure the tank is well aerated, and do not raise the temperature beyond around 28oC (82oF).
Due to the life cycle of the parasite, the whole tank must be treated, in order to kill the parasites which are not attached to fish. Therefore it is not appropriate to treat only the affected fish in a separate isolation tank.