Flatworms, wondering whether to treat or ignore.

[Chin_Lee]

I've used flatworm exit on my tank twice now. The first time i followed the recommended dosage of 1 drop per gallon and lots of them died but a few days later, i still see them around. I retreated at 2 drops per gallon and more died but a few days alter, I stilllll see them around. I haven't treated again since and they are starting to come back. i have the reddish ones that primarily stay on the sand close to the rocks.
both times i had no side effects on any anenomes, sps, lps, inverts and i didn't do any water changes after either.