I tried to do nothing about my flatworms, I waited about 8 months,thinking they would eventually die themselves out, but no luck.
They multiply by splitting, and they are photosynthetic, so it basically means there is no stopping them from multiplying. They smothered my GSP, and thats when I used FE. I couldnt even see some rocks at all, most of the front of my glass, was all ruct-red from those things
I used 3 times the recommended dosage (no way I was doing all that work again a few days later just to be sure they were dead). Im proud to say that every sinlge flatworm is dead, and I didnt lose any livestock at all. just big time water change, and frequent changing the carbon, and no problem. Everything was receded and shriveled for a couple of nerve-wracking days, but on day 3 things started opening up again.