You have too many organics in your tank. I had the same thing happen to me. I had about 40ppm nitrates and to take care of the high nitrates I cut back from feeding 2 times per day to 1. My nitrates are now 0 and my flatworms are also all gone. I tried flatworm exit 3 times and it killed most of the worms, but they always came back. Also flatworm exit takes it toll on your livestock. It took about 3-4 weeks for the flatworms to die off. I'd try this first before doing the flatworm exit.