Ichthyophthirius multifiliis, commonly known as ich (sometimes spelled ick) is as ubiquitous to tropical fish as the common cold is to humans. The comparison ends there, however, since the common cold is caused by a virus or bacteria, and ick is caused by a protozoan. This tiny organism is a free-floating parasite that spends most of its time looking for a host, or victim. When it finds one, your pet fish, for example, it attaches itself and flourishes.
Ich sometimes is referred to as "white spot disease," since it causes a breakout of white spots all over the infected fish's body. The protozoan travels about the aquarium searching for a host. It then attaches itself to the fish's body, where it finds nourishment until it forms a white shell, the visible spots, and then grows big enough to drop to the bottom of the tank and multiply itself by the hundreds, issuing juvenile organisms, called tomites, into the water to begin the cycle anew.
Stress plays a big part in making your pet fish sick. Fish are stressed easily when moving from one tank to another, contending with new tank mates, adjusting to changes in water temperature, dealing with dirty aquarium water, and over- or underfeeding. Like humans, fish have a strong immune system, but when they are undergoing a stressful time, their immune system suffers, leaving them vulnerable to all kinds of diseases stemming from parasites and bacteria.
In addition to stressing out the fish, overcrowding causes ich because it raises the ammonia levels in the tank, which makes the environment more welcoming to ich. Ich is present naturally in almost all fish tanks in manageable levels. The more fish you bring in, the more ich comes in with it and the potential for an outbreak increases. Since the disease isn't visible until it is underway, you easily could put carriers into your tank inadvertently. In addition to overcrowding, always be aware of which species of fish play well with others. Some fish, such as angelfish, are bullies, and having them in a tank of gentle fish, like guppies, will stress out the guppies.
Water quality and temperature plays a big part in helping ich spread through the tank. If the water is dirty, or if the chemistry is unbalanced, you are putting out the welcome mat for ich. Check water quality with test strips to be sure the levels of pH, nitrites, nitrates and ammonia all are well within acceptable levels. Keeping the tank relatively warm, over 75 degrees Fahrenheit, will keep fish healthy and ich at bay. Always place sick or injured fish in a hospital tank to isolate them from healthy fish, and maintain your tank aquarium and water-change schedule to keep ich from taking over your tank. Never subject your fish to drastic changes in water temperature.