From what he wrote to me in an email, most people don't do large water changes very often, so, the fish are not used to them. I've heard many times of people doing a water change, after a long period without them, and losing all of they're fish. He said, and don't quote me, I didn't look up the email, but he said that I should have no problem with the 100% water change, because I do large changes every week. I'm sorry to hear that all of your fish died. Callamanous worms have killed some of mine, and some when I medicated. On the bright side, as long as your local fish store didn't sell you infected fish, your tank is clear of the parasite, and the new fish should have no problem. Sorry, though.