Flatworm Exit is awesome stuff. Don't be afraid to use it! Your flatworms will be toast. Remove your carbon, leave the skimmer on (and watch it because it might become very active), use 1.5x the recommended dose, remove the dead flatworms with a canister filter or filter sock on the overflow. After they're all dead (I wait about a half hour) use lots of carbon and do a large water change. Voila! No more flatworms. Your corals, fish and invertebrates will be more bothered by you fiddling with the tank than by the Flatworm Exit.