He has ich in his tank. The new fish are more susceptible because they're stressed. The only way to get rid of it for sure is to fallow the main or hypo it if there are no inverts. Otherwise any time a fish is susceptible to it, it will present again. If he QTs the purchases before putting them into his tank he may ease the stress of transition (especially if he can match parms with the main prior to adding the fish in). Maybe that way he can continue to live with it the way he is. Just know he has a ticking time bomb. So do I. I'm living with ich in my main, but so far nobody is symptomatic so I just continue to watch them. If I have an outbreak they'll all be properly treated (regardless of who's symptomatic).