I recently eradicated mine, however I had a very small amount of them (less than about 25 visible, maybe only 100 in the whole tank).
I dosed flatworm exit immediately upon site of the first flatworm. I believe some eggs came in on a zoa frag I bought, because all the flatworms were congregating in this area, and all of them were small and not full grown yet.
Before dosing, I changed the carbon in my reactor and installed 2 additional carbon HOB filters right on the front of the display tank. You want to get carbon running immediately when the flatworms start dying. The medication itself doesn't hurt anything in your tank (other than flatworms of course), but the flat worm bodies release toxins that bother everything, not to mention the ammonia spike that occurs from a mass die off.
Getting multiple carbons filters all going at the same time seemed to help a lot. Within about half an hour after dosing my male occelaris clown was acting funny, swimming all over the tank in the dark and never settling down for the night. Normally it sleeps in an anemone with its mate. I was a bit worried at this strange behavior. But by the next day it had calmed down, and everything was back to normal. My corals started opening up again and I haven't seen a flatworm since.
I believe the key in this case was the early detection, so that their numbers hadn't already grown to plague proportions.
If yours have already grown to plague proportions you don't have a lot of options. The best things you can do are:
reduce your photo period and intensity, this will slow their reproduction.
build a flatworm vacuum and start using it dailly
to do this, attach a plastic straw to some flexible tubing and put a mesh bag on the end of the tubing. Place the bag in your sump (or in a bucket if you don't have a sump). Start a siphon out of the display tank and suck up all the flatworms you can see. They will get caught in the bag and you don't lose any water because it flows right back into your sump (or into the bucket). Rinse out the bag and repeat again the next day.
After vacuuming as many as possible out day after day, you can consider using flatworm exit again, but only after you have removed as many as physically possible. many of them will still be living in the rock and out of site.
When you use flatworm exit, just use the recommended dosage. The goal isn't to kill all of them at once, but just to reduce their numbers. You'll want to dose the recommended dosage every other week or so for maybe 3 or 4 doses. Killing about 50% of them with each dose.
Once you have their numbers drastically reduced, then you are safe to use a bit higher dosage, maybe 2 times the recommended to get all of them. Using 9X the recommended dose like you mentioned above is just asking for trouble in my opinion.
After dosing, be sure to run fresh carbon and do some water changes. Nothing drastic, just 15-20% at a time. Its better to do 3 or 4 20% water changes than to do 1 50% change.
Edited by speyside712 - December 05 2017 at 8:33am