i was looking into my 10 gal tank and noticed there was a black worm about 1 -2 inches long slinking across my aquarium . its flat on top and has a rounded mouth
hope it's a planeria; a type of flat worm. You can tell if it has cross-eyes on it. Otherwise, it may be from your description of the round mouth it could be a small freshwater leach. If it's a leach, i would kill it ASAP because those things are parasites that suck on fish blood.
im not sure i googled both and checked the videos and it looks like neither when it swims . its movements seem slower waves and it locks onto plants and raps around them like a snake.
it could be out my pond as i added a plant from there and that also added these small insects that seem to be doing no harm its just this thing that worries me.