I had a big problem with flatworms in my reef aquarium - key word there is
had. I used a product called Flatworm Control - I'm sure it's the same thing as Flatworm Exit, but by a different company. I siphoned the ones I could get at each day for a week straight, then I nuked them with the flatworm control. It took very little time and they all were dead. I did a water change the next morning and all was good in the tank. I have several Acros, Montiporas, Alveopora, elegance, plate - long and short, Xenia, Miliporas, mushrooms, hammer, worm rock, zoos, you name it, I've got it and only one colony of zoos showed any sign of stress from it. That colony was fully open within 24 hrs later. Guy at the LFS told me not to waste my money on the Blue Velvet Nudi, as you need one per each 10 gallons of water in your aquarium and he has never seen them actually do the job before all perished. Good luck!