I had a big problem with them in my 90. Find Flatworm Exit (Salifert). I only found it online. I dosed the recommended amount for about an hour, then ran carbon for a day to remove the toxin they release upon dying. The next day, there were a few survivors, so I very heavily dosed the tank again, but there were so few I didn't need to run carbon again. Two months later... not a trace of flatworms, and no negative consequences of the treatment (well one, my Regal got cloudy eye, but it cleared up after a few days.) I would definitely use this product again.