The ICK can be residing in your tank and your fish are healthy enough to fight it off and when introduced a new fish into the tank it gets lambasted for lack of a better word. Or the place you are buying fish from are not very good pet keepers. Some LFS's don't care about their customers tanks and just want to sell fish and believe me they sell them fast these types of stores 2 for 1 sales and all the goodies. You have to QT and feed garlic and selcon for 2 weeks minimum and fresh water dip before going into QT and after the QT helps the fish get ride of any stragglers.

Bill