FWIW,
It seems I was able to mostly eradicate my flatworms by adding a species of Sargassum shrimp from the Gulf of Mexico. There are several different species I am told, so I will not say which I have. The genus may be Latreutes, but I am not sure.
They are hard to find as I know of nobody commercially selling them - I got them shipped directly from an individual in TX through a fella I know. They are tiny - less than 2" and very cryptic. Nonetheless, 10 days after I added 4 of them to my tank (90 gal), my flatworm population was decreased by the THOUSANDS. No kidding. It is really hard to believe that these tiny shrimp ate them all, but nothing else changed in that 10-day period. It is possible, but very coincidental, that the large population crashed on its own. Still, I had them in large poulations in my tank for over 6 months and the numbers kept growing and growing - to the point where I just stopped siphoning them. Two other fellas tried them with me and unfortunately didn't have the same results I had. I can't explain it, but I just wanted to present my case. I know how staring into a tank of flatworms can deflate your satisfaction....