If you encounter an infestation of flatworms in your reef aquarium, spefically those of the Acropora eating variety, you will never full get rid of them unless you take drastic measures. Sure, you could try your luck with a sixline wrasse or some other flatworm predator. Or you could take out corals and dip them regularly. You could even try an in-tank treatment that will weaken or kill the flatworms. Unfortunately, these are not effective long-term strategies and the worms will keep coming back. The only tried and true way to fully rid yourself of the nuisance is to remove every Acropora coral from the aquarium and place them into a separate quarantine tank for treatment. The flatworms in the aquarium will need 6-8 weeks to die off due to starvation, and the affected corals can be treated with a variety of dip treatments while you wait. Once it has been determined that all of the corals are flatworm free, then they can go back into the tank, though I would do it very cautiously and would probably try a few test subjects before putting all of my Acros back in.