Read up on SW ich.
Spots will appear then drop off but the ich is never really gone just moved along in it's lifecycle, nor is FWD alone a cure.
Seems a lot of us probably have ich in our tanks but often never shows as fish can handle it if they're kept under good conditions. Having a couple of Tangs in a small tank might have caused enough stress to allow the ich to take hold.
Might also be a factor is setting up a new tank then quickly adding stock you got a little cycle going (spike of ammonia).
|