Had the same problem for months, tried everything, after reading another post, used Quinine Sulfate(quinsulex, made by aquatronics) 1 capsule per 20 gallons of water, turned skimmer off (remove any carbon, poly filters, etc.) within 12 hours all signs of flat worms were gone(very bad smell with the die off), would sugest removing any sps corals, all others should be fine(mushrooms, leathers, lps might close up some but should be fine) after 12 hours I turned on skimmer, added carbon and a poly filter to remove the medication.(also did a 10% water change) did the treatment 11/7/02 corals are back to normal today and no flatworms at all !!! got to love it!