Help! My fish have the Ick!!! Or, as a customer once told me, "the Yuck". After a momentary chuckle to myself, I went on to explain that the common term for this common aquarium fish disease comes not from some nasty appearance the infected fish take on, but from an abbreviation of the disease's scientific name.
Ichthyophthirius multifiliis is the parasite responsible for the freshwater version of the disease. Cryptocaryon irritans is the saltwater version most often seen. But the term "ich", as it is more appropriately spelled, has come to be commonly used whether talking about fresh or salt.
In any case, the story of "ich" goes like this: Healthy fish carry the parasite in a dormant state without suffering any apparent ill effects. Fish that become stressed or weakened lose their "immunity" and begin to show signs of an evolving infestation. Initially, the fish's fins appear to have a few, small white nodules about the size of a grain of salt on them. Irritated, the fish will "scratch" up against rocks and decorations. Over the course of a few days, the spots multiply and spread to the body as well. The disease will also spread to other fish in the tank, even to those who are not stressed or weak. Left untreated, most if not all fish in the tank will die, if not directly from the parasite, but also from the secondary bacterial infection that typically sets in at the parasites' attachment points.
It's important to know, before treating, just how the medication works against the disease. While still on the fish, the parasite is not affected by common ich medicines. After a number of days, the parasitic cyst falls off the fish, and, while unattached, goes into a reproductive phase. Multiple offspring from the single cyst are then released in a free-swimming form which reattach to the fish-- both those previously affected and those not-- and the cycle starts all over again. It is in this free-swimming stage that the parasite is most vulnerable to the medication.
A common strategy for ich treatment includes increasing the water temperature of the tank by several degrees. There is a drawback to this in that additional stress is introduced by the change in temp at a time when fish are already stressed, and in that warmer water holds dissolved oxygen less easily. The benefits outweigh the drawbacks, however, as it is typically a sudden drop in temperature that caused the stress in the first place, and additional aeration can be employed while the temperature increase is in effect.
The reason for upping the temp is to actually cause the parasite to reproduce more quickly. The rationale is that the medications typically have a short period of activity-- re-treatments are often necessary before a cure is complete. If the parasite is more frequently in the stage of its lifecycle where it can be acted upon by the medicine while the medicine is active, the more parasites that will be killed.
Interestingly, in the saltwater version of the disease, there's a technique used to improve treatment success, and one that can often nip early active infestations in the bud. It's called the "freshwater dip". This trick takes advantage of the fact that the parasite, even when attached to the fish, is actually quite a fragile organism. The freshwater dip involves making up a container of de-chlorinated and tank temperature-matched freshwater. The fish is (hopefully) gently netted from the tank and placed in the freshwater for a short period-- usually only a minute-- before being returned to the tank. That sudden change from salt to fresh water, though, causes an "osmotic shock" which, while only minimally stressful to the fish, causes the parasitic cyst to literally explode.
It's important to monitor your aquarium daily for signs of disease, and for any changes in the environment that may cause the stress that leads to disease.