Tough breaks man! What kind of hydroids are you seeing? Colonial or digitate? I have always had colonial hydroids in my tank. Tried fenbendazole treatments multiple times (gets rid of them for about a month and then they come roaring back), manually removing, and smothering them with epoxy. Nothing has worked for me. I’m just letting them be now and trying to not worry about them and they seem to have found a balance. They are there and they annoy me, but I don’t think they are getting worse anymore. Anyways good luck and don’t over react to the hydroids would be my advice. Sorry to hear about the acans though. Any problems from the ato and salinity swing?