I have found the most effective way to treat is is to raise the temperature of the tank to 80-82 degrees (slowly, one degree every hour or two) and add salt to the tank at a rate of 1 tablespoon/5 gallons. You have to treat the main tank, not a hospital tank, or when you have treated your fish you will just be re exposing them to ick. Treat them until you haven't seen spots for 10 days.
Jan