Now that you have them, the populations will increase and decrease depending on the available nutrients. Some wrasses and other fish will eat them, you can also use Salifert's Flatworm Exit. Do a search on flatworms, specifically red planaria.
It is a manageable problem. It could be a lot worse as far as pests go. Flatworm Exit works if used properly. And as stated above, there are many wrasses that will help control the population (and possibly eradicate the issue all together).
+1 to the previous posts. Flatworms aren't bad in reality. They become bad when they have reproduced to the point that they are on your coral. This is a problem because it won't let the coral get the light it needs therefore receding. Get a wrasse or do the flatworm exit. I am currently dealing with them as well. I used the FW Exit already and it worked I just have to do it again and that might be the case for you as well.
The flatworm control appears to have worked. It has been maybe 16 hours since I dosed 4 drops per 10(ten) gallons. I'm not sure if I should follow up with a second dosing or just wait to see what happens.
Update:
They did return about 5 days later. (I never did the follow up dose).
I dosed on back to back days this time now I'm just waiting. No I'll effects on the tank except my yellow tang seems to breathe more rapidly immediately following the dosage.
How is the behavior of the sixline? I've heard conflicting views.
I have used prazi pro in reed tanks and it works great u can watch them shrivel and die.. But you must have zero phosphates in the water when using any method. Just before the flat worm dies it secrets a fluid which is highly toxic when present with phosphates
I'm doing ware changes, but not dosing carbon. This tank is only set up with slimmer, heater, and powerheads. How would I dose carbon?....put it in a mesh bag and sit it on a frag rack and tank?