If you want to catch them blanch a piece of squash and put it in before the tank goes dark. They will be all over it especially once the lights go out. The ramshorn snails in my tanks are the most aggressive of all the snails but don't seem to harm the plants. I don't know about the flukes some can be a parasite. Can you see them? I have a shrimp tank with no fish a I see all kinds of weird stuff moving around, no problem yet. check this out. http://planetinverts.com/what_is_that_bug_in_my_aquarium.html