I just got through a severe red flatworm infestation, and it was a rather traumatic experience. When we first noticed them I tried a six-line Wrasse and an Arrow crab, both recommended by my LFS. We also tried siphoning them out, but there were just way too many of them. There was no visible impact or reduction of the infestations with any of these remedies.
Down a separate thread, we added some Chemi-Clean to control some algae, the prescribed dosage for a 120g. Well, by the next morning all of the coral had gone into shock, were shut down and sliming like crazy. After adding carbon (bagged) to the refugium and three 25% water changes over the next 4 days, things started to return to normal. The flatworms are gone, but there were a couple of fatalities - my clam, a cleaner shrimp (although one did survive), and a couple of heads on the Sun Coral. All of the fish survived.
What happened was that the huge die-off of the flatworms released an overdose of toxins into the water, which is a known reaction. We refrained from using Flatworm Exit for that exact reason and were totally unaware that the Chemi-Clean would impact the flatworms.
Depending on how bad the infestation is, the flatworms really didn't seem to have an impact on the ecosystem of the tank, you could try Flatworm Exit or Chemi-Clean, but be ready to respond quickly to any reactions in the tank.
There are still a few flatworms wandering around, but should we should be better able to manage them going forward. Oh yeah, the algae is also gone!
Good Luck!