I wouldn't get a fish, use flatworm exit. I was nervous about using it at first when I had flatworms but I tried it and it was wreaking havoc on the flatworms within a couple minutes. It was pretty amazing, all you have to do is follow the directions pretty well and it'll alright. Try siphoning out any flatworms at first, add the proper of FE, let it sit, watch the flatworms go nuts, suck them out, and then do a water change.