Other than living in a sterile environment you cannot get rid of all bacteria nor would you want too. 99.99% of bacteria is either beneficial or of no harm. Right now you literally have millions of bacterial cells growing on your own body right now. It is pretty much impossible to answer your question unless you sent a fish in to a fish pathologist to isolate and determine the real problem. The reason no one else has answered is it would just be guess work at best.
-john