I've read all the comments (or most of them) about hydra not posing any "danger" to fish, BUT I found them to be really unpleasant to see whenever I looked into my 9-10 month old 20G tank, so I was determined to get rid of them. So, based on a number on-line favorable comments/recommendations, I ordered some PLANARiA ZERO from Japan. It took about two weeks to arrive. I followed the instructions and BAM!, at the end of the 5-day treatment I had zero hydra. None, nada, zip. Great product. I was, and am, a very satisfied customer.......