it caqn be tough on any inverts you have......
Its not the flatworm exit that hurts its the toxins the flatworms release when they die.
Siphon out as many as you can see, dose the FW exit, siphon out any that appear as they die and have a 25% water change ready.
Get some wrasse
Get a Yellow Coris Wrasse. They're inexpensive, behave nicely, and pick at flatworms all day long. More so than a six line.
I do have a few prized inverts I'd rather not part with.. do you think that just lowering nitrates will really help?
if you have a holding tank for corals etc.. i would transfer most of your things to there and use flatworm exit. thats what my buddy did... also kills off bristle worms he said