PBTs are ich magnet. I have been living with ich for years. It comes and goes especially when the fish are stressed for whatever reasons. In my case, temp fluctuation around the spring seems to bring it on. I learn to ignore it. As long as my tank is health, the ich goes away. I used to run a UV which seen to help. The UV has not been running since last summer. I have seen ich on my PBT and chevron since but they went away by themselves. If I took those tangs out at the time, no one would ever knew that the tank had ich since my other fish (regal angel, blueface, GSM clowns etc) are more resistant to them and had no signs of ich. Even though you don't see ich on your fish, it doesn't mean they are not in your tank.