How does my fish tank get ick in it when the water is in pretty much perfect condition..?

I have a 55 gallon freshwater fish tank and just recently realized that a few of my fish are covered with ick. I have not added any new fish for a few weeks, i have tested the water condition for Ammonia, nitrites, and nitrate levels and each one is where it should be. My room ate did however put a few of his fish in my tank when his ammonia level was really high and wanted to keep them in my tank till he got his ammonia level under control. Not sure if maybe his fish could of brought it in and gave it to my fish or at least made the ick worse in my tank. I did put a recommended dose of salt in my tank as well as one does of "ick away" by Wardley. This is my fist run in with ick, well that i know of anyway. I just want to know if my fish are in jeopardy. I have a total of 5 Grommies,2 Angel fish, 1 Pacu, 2 Striped Raphael Catfish, and 3 Black Skirt Tetra's. Just want to make sure i am doing everything i need to do to correct this problem.