I had flat worms for about three years. Did multiple doses of FWE over the course of three years. I even pulled all the rock and soaked it in SW with FWE and blew all over the rock with a power head and they came back. When I moved from Yellowknife to Calgary I had the rock in tubs of saltwater with heaters and powerheads running off an inverter with a triple dose of FWE in the water in the back of my truck for two days. They still came back! I finally gave up with the FWE, it did kill them but not every last one. I started to do regular water changes and sucked up as many as I could. I did buy a yellow wrasse to see if it would eat them. I never saw him eat any but they eventually did go away, kind of just died off. The yellow wrasse sleeps in the sand bed. Freaked me out one morning when I saw just its head sticking out of the sand bed. I thought something ate it and left the head laying there. Until I noticed the gill covers moving. Neat watching it circle it sleeping spot in the sand then burrowing in. Kind of like a dog lying down to sleep. Now I am off topic and will shut up.
Rob