Hmm, yeah, I've read good stories and very bad stories about peoples experience with Flatworm Exit. They're plain ol Acoel worms. Not harmful, in fact some what beneficial algae eaters. There's just so many of them. Sounds like the best thing to do is keep their food to a minimum and suck em out at
water change. I've had a lot of build up in gunk in my sand lately. My water quality is good but there's a lot of organics built up in my sand. All the worms stay fairly low. I also found I could stick the sprayer off a spray bottle in the tank and "dust" off the rocks while I clean. I always have trouble cleaning out the snail "crumb trails" and other stuff in the nooks and crannies. I think if I just get that under control it'll go a long way to slowing the worms. They can stay as long as they stay under control. I want a Green Mandarin and I've heard they might munch on them, so that may help too.