well, ich is a parasitic infection usually observed through small pin-like white specs all over the fish. It spreads fast and can lead to death if untreated. It is a rapidly spreading infection that once present in an aquarium, can spread to all other inhabitants very quickly.
There's a couple of ways to deal with it but the preferred way is to quarentine the fish you have that are infected...which will most likely be all of them....and administer hyposalinity treatment(aslo just called "hypo"). This is basically when you lower the SG of your quarantine water slowly(over the course of several days) to about 1.008-1.010. At this SG, the parasites will die after about 4 weeks of this treatment. This is the prefered way because it's much less stressful on the fish, it's natural, and it's success rate is really high.
Your first step will be to set up a QT(quarantine) tank and then start hypo treatment. Depending on how many fish you have, this could be a difficult task. Please remember that lowering the SG in your main tank will most likely kill off inverts and corals.
Here's an article about setting up a qt tank:
http://www.aquariumadvice.com/article_view.php?faq=2&fldAuto=17
also, if you need more info on ich, just use the search function. Ich has been covered at length on this forum. One last thing, be aware of quick fix medications and chemicals. Many have poor success rates and will stress the fish. The only chemical that is proven to work well is copper.
