One important thing will be to properly ID what kind of flatworms you have. If they're Acro Eating Flat worms, they can be a scourge!
If they're Red Planaria, they really aren't that big of a deal, though they reproduce quickly and CAN reproduce to the extent that they can COVER your corals, not allowing the corals to receive enough light. The only other time that they cause problems, is when people treat them, chemically, without realizing that the dead release toxins.
If you have a small population, manual removal is the best option, along with increasing flow. They don't like high flow. A lot of wrasses will consume them, such as Melanarious and/or 6 lined wrasses. You can siphon them out, rather easily and they'll tend to congregate in low flow areas.
When considering if you have a small population or not, keep in mind that there are usually MANY MORE than you actually see.
Salifert Flatworm Exit works great and is reef safe. However, read the directions very well. You can triple the suggested dosage, without any problems at all.
The MOST important things to consider are:
Flatworms become toxic, when they die. There's many many more than you see. If too many are left in your tank, dead, their toxin can crash your tank. For this reason, it's CRITICAL to remove as many as you can, by siphoning them out, BEFORE you treat. When you treat, they'll start dying, almost immediately. They'll be floating all over the water column. Use a brine shrimp net, to scoop them out, as they're floating around. Use a turkey baster or power head, to blow all of your rocks off. This will suspend any dead worms that are in the rocks.
Again, the MOST CRITICAL aspect of treating for flatworms is to remove as many as you can, before treating and then to remove as many dead ones as you can, during and after treatment.
