Ich is a very common problem with new and old aquarists alike. Ich is a parasite that is always present in aquariums, but can't always infect the fish. Fish can catch ich when they are very stressed. Once the ich parasites burrow under the fish's scales, they begin to reproduce, and the increased number of ich larvae in the aquarium infects other fish.

If you don't know already, ich appears as small white spots on a fish (Ich is often called white spot)

There are many ways to kill ich, but the way I kill them involves heated water, salt, and/or aquarisol. When the water temperature is increased, so is the ich's rate of reproduction and therefore, the lifespan of the ich itself is decreased. More larvae are in the water, and this is when you attack with your weapon of choice (salt, aquarisol, or any other number of anti-parasite fish meds). The larvae are killed off by the treatments, so less and less infect the fish until their are next to none left. When the larvae cannot survive in the water, no more can infect the fish, and the remaining ich dies off, leaving the fish healthy once more.

The ich still is in the water though, and if you aren't careful, it will come back ;)