They are not *always* a sign of overfeeding, though. I have a
QT/starter tanks for my plants and I get planaria outbreaks in it all the time. I don't feed that tank (except for flourish, ferts, and excel) so they can occur just from having plants. They eat plant material, and can grow, thrive, and colonize just from having plants around.
The Otos may eat them. I know Corys and guppies thoroughly enjoy the little things. Scarlet Badis are great little planaria hogs as well.
Gravel vacs, water changes, and cutting back on feeding will help. Even if they didn't come around from overfeeding, cutting back on potential food sources will cut the numbers.
I have also read about using dog dewormer in tanks to eliminate planaria and hydra. I'm not bothered by the planaria enough to try it, but it has had good results for other people. Here's a link to the aritcle.
http://www.aquaticplantcentral.com/f...imp-tanks.html