Are we talking little red flatworms (2-3mm in size, rectangular, and crimson-colored)? If so, wrasses will keep the population in check but they can overwhelm your tank and die from over-population. Each one that dies releases a small amount of toxin, and when thousands die they nuke your tank.
A SHORT-term solution that you can do routinely is siphon them out with a piece of hose. Simply get 6-7 feet of small airline tubing and an empty bucket, stick one end in the bucket, the other end in front of a powerhead to force a siphon, and then use the hose to vacuum out the little buggers. This worked great in an afflicted frag tank I had a year ago.
THe only permanent answer is to use FlatwormExit. This will rid your tank of all of them but you have to keep in mind they release toxins when they die, so the safest course of action is to remove any CUC and anemones from the tank, siphon out as many as you can see, dose 1/2 the recommended dosage, wait a day, and do a
substantial water-change, repeat for about a week. Otherwise, once you get them you aren't
ever ridding yourself of them and they breed like Frenchies at a bakery