Thanks guys.
What's very interesting, and I don't defend the sensibleness of this, necessarily ...... this FOWLR is a separate system from my reef tank, but when I do water changes in my reef tanks, I flow the 'old' water through this FOWLR (no possibility of back-contamination though). I know my reef tank has ich, and has had for close to 2 years, even though fish are essentially asymptomatic. Yet, fish in the FOWLR clearly show ich symptoms. Friend of mine was always convinced that reef tanks are able to 'supress' ich in a way a non reef tank could not. Not proposing anything, necessarily, just a curious observation.