HiGuppy has given excellent advice; a salt treatment works wonders in combating Ick.
It is my intent to add clarity and strengthen some of the points already made by Guppy. I get many phone calls on fish health issues and no mater how I explain the treatment some people, especially those new to the hobby, take shortcuts; for example they just dump in some table salt and hope for the best. As mentioned it is important to raise the temperature to 85 - 86 degrees to speed up the parasite's life cycle. If you do not already have an air stone bubbling in the infected aquarium it is advisable to add one as the dissolved oxygen level drops significantly when the temperature is raised and yes the fish can suffocate from lack of oxygen.
Unfortunately many plants do not do well with this salt treatment and may appear to be failing but will usually come back in time. Removing them to a salt free environment after a thorough rinsing may save them; however they must be kept at the high temperature as well. When the Ick capsule breaks up in the plant holding tank the small parasites will be unable to find a host and will die within 24 - 48 hours. Ick is easily transferred to other fish tanks so do not share nets, heaters and wet hands between infected and non infected tanks. Fish in treatment appear to do better with a reduced feeding regiment.
When you look into the pet shop / fish store aquarium their fish may appear healthy without any signs of disease. Ask how long the fish have been in the store and when the last time new fish were added to that tank. If it has been 2 weeks or more your chances of getting disease free fish improve.
Impressive looking Clown Loaches and young Oscars are notorious for getting Ick after they are home in your tank. These and many others can tolerate salt. If you are unsure about your fishs tolerance for salt look for answers in a reference book or ask an expert.
A salt test kit, available at your local fish store, will help you get the exact dosage. Something in the range of .2%, is where you want to be. For large aquariums that works out to about 2 pounds or slightly more than 2 cups of salt per 100 gallons. The salt I use is "Evaporated Sea Salt" available at Home Depot in 50 pound bags. Do not use salt with iodine added or water softener salt with "YPS" or any other chemicals added.