hydra dont attach to the fish itself, just to solid surfaces, where it uses its tentacles to catch food to shove into its mouth.. if you had an extra tank to put the fish into for a few hours i would, then strip that tank down and clean it. but, like i said-hydra wont harm the fish in the long run, and it would be a PITA to break down a tank and have to recycle it. uggh. its too much work.
like someone else said, scrape them off the surfaces, and net them out with a fine net.. and cut down on their food source.
Thats not a problem since all the bigger ones died,but the monsters are in my big tank,there is only the megas an NTT a blockhead and a senegal so il get some small ones tomorrow.