Unfortunately, fish are susceptable to a variety of diseases, especially in the aquarium, due to the close proximity of other fishes. The most common disease encountered by fish is ich (pronounced ick). Ich, also known as white spot disease, is caused by the parasite Ichthyophthirius multifiliis.
The ick parasite is round, covered by cilia (hair-like extensions), and about 1 mm in diameter and so it is visible to the naked eye.
The ich parasite burrows its way into the skin of the fish and feeds off the fish's blood. The gills may also be affected. In an ich infected fish you will see small white spots on the fish. The fish may act ill, with fins held close to the body, and you will often see the fish glancing off of rocks, presumably to dislodge the parasite, as they are irritating to the fish. If the gills are affected your fish will gasp for air.
After a few days of feeding on the fish's blood, the parasite bores its way out of the fish and forms a cyst in the tank. This is the reproductive stage, and when complete, about 1000 free-swimming young parasites are released. These are much smaller than the adult stage and are not visible to the naked eye.
This free-swimming form then seeks out a host, burrows into their skin, and the whole cycle begins again. In addition, because of the small size of the free-swimming stage, they are easily able to enter the gills of the fish. By the time you find white spots on the fish's fins and body the gills are usually already heavily infected with ich.
There are treatments available. Treatments work only for the free-swimming stage. So don't expect the white spots to disappear off your fish immediately. You will have to wait for these to fall off, form the cyst stage, and when the free-swimming stage emerges they will be killed by the treatment. While you are treating your fish for ich (described below) it is a good idea to turn the temperature in your tank up a couple of degrees. The ich life cycle moves faster at higher temperatures. In this way you'll get the cysts to the free-swimming stage much quicker and can kill them off.
It is important to treat the fish because ich can be fatal. It can also lead to secondary bacterial infections due to the irritation of the fish's skin.
If you go to any pet store that sells fish, you will find a wide variety of different brands and treatments for ich. Some of these seem to work better than others.
If you read the ingredients you will find that not all of them are using the same chemicals to treat ich. Some are formalin based, some contain malachite green or methylene blue, and some contain copper sulfate.