Its very hard to get rid of them in my tank now without breaking my acros.
Especialy the ones by the montipora digita. I suck mine out with the hose which seems to be working very slowly. I brought a sail fin tang which did nothing. My rabbit fish has a knibble now and then. I stopped popping them about 6 months back which has helped on the way to there doom.
As for the water quality being the reason of there arrival, i think this is not true as i have had some bad tanks over the years and the only time i have ever got them is from someone elses coral. Frag or shop.