Here's my weird flatworm story: What I had was definitely flatworms but they were not out of control. Got FlatwormXit and accidentally -way- overdosed the stuff. Did 50% water change. The next day they are still there. Overdosed my tank again with FlatwormXit, did the water change. Did three treatments in all and they are still there !
I talked to somebody knowledgeable and he said many flatworm species are benign meaning they are harmless. A week after the last treatment they are still around but not spreading and a month later they are just gone and they never came back and that was summer before last! But maybe in your case its better to be safe then sorry.