Originally Posted by sphelps

All my tanks have had flat worms at some point and I no longer bother trying to treat for them. Flatworms are harmless and rarely cause any issues even if left completely uncontrolled. Treating them with exit usually results in more trouble as dead flat worms have to be removed manually after treatment and they almost always come back. Multiple treatments can sometime lead to the worms building up an immunity. Best control is a wrasse or something that eats them, a six line is usually a good candidate for almost any tank.