I don't get it either. Everything I read says it's poor water, but my water test are all good. My tank is understocked and I am really good about water changes.
I will try to post a video, but I am not sure that I know how.
I was able to locate an imaged online that resemble the flat worm we were talking about. Here's the link: http://www.planetinverts.com/what_is..._aquarium.html for more info along with some other critters (in case you have some other bugs in your tank you were curious about). I still don't think they are planaria, because my other tank just broke out with some and they look like the stereotypical planarias. Maybe there different breed of planarias, who's knows....?