One thing to remember is that UV sterilizers can only sterilize what they come in contact with, which is the water. Parasites can lye dormant in the substrate or on a fish for long periods of time until stress suppress a fishes slime coat layer. Even with a huge UV running, the source of stress has to be eliminated for the ick to go away. Sometimes very frequent, large feedings can help boost a fish's immune system even when the stress source is still present.