From what I understand Hydra will not harm larger fish but will eat fry. I had a friend who just had a problem with them in one of his breeder tanks. He used the raising the water temp to 100+ and salt to kill them. Unfortunately you have to remove the fish to do that. I also heard paradise fish will eat them up. Some people used to use a 9 volt battery with two leads to kill them. Look up hydra and compare them to some pics on the web. Here is one I found with a quick search. http://tbn0.google.com/images?q=tbn....ca/16cm05/16labman05/lb5pg3_files/hydra2.jpg
I hear it is a pain either way.