I don't know what flat worms are, so its just a guess on my part. Anyway, the biggest one I took out of the tank tonight was black, about an inch long, it looked like a slug or a snail without a shell, and two small antenna on the head. the other four I took out of the tank were small, also black and only about a 1/4 inch long and no antenna, kinda hard outer shell on the back and soft under side. I have in the past taken about 4 or 5 out of the tank like the last ones I described except they were white. I have also taken one out of the tank a few months ago like the first one I described except that one was lime green. Up untill tonight I have only seen about 5 of these things in the tank since we set the tank up, tonight I took four out and missed one. Do I have a problem and if so, what do I need to be doing if anything to correct it.