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....?
ps Insert video viva youtube link