My vote is for flatworms as well. I used a diy airline tubing siphon and could never get on top of them. I finally broke down and used FWE at 1.5 x the suggested dosing twice, a few days apart and that got a goodly number of them, but they came back anyway. You MUST be prepared to do a very large water change and run lots of carbon because of the toxins released. A pair of Blue Star Leopard Wrasses, on the other hand, have completely rid my system of them, and they continue to be a pair of my favorite fish. Very colorful markings, always out and about and on the hunt for little pests.
hth