Only thing about the sixline is, won't it only help control the population of them? It won't get rid of them fully will it? I don't know. With all the holes in the rocks, you'd think the flatworms would still be able to reproduce. So, if you get a sixline, it might eat most, but then once you remove it, they will come back? Guess you could get something to clear most out and then use the FWE....but you said you don't want to put any chemicals in your tank. FWE didn't get rid of all of mine. I did 3 treatments and still seen 1 or 2 on the glass. The sixline is eating them though cause that one that was on the glass, I watched him pick it off. And all day he hunts the rock and picks at it.
Steve