Sounds like planaria and yes - they usually start showing up in large numbers due to overfeeding & excess food ending up in the gravel. They aren't bad for the fish, and may even be food for some types of fish. I'm guessing that the plecos may have been eating them in the other tanks. Maybe a good gravel cleaning may help, as well as the plecos that you've introduced into the tank. Not feeding the fish would help, but avoiding overfeeding may be better. It's difficult not to overfeed as the little guys always seem to be hungry, but it has to be done.*
Numerous articles on planaria in aquaria can be found in a search - this is just one:
http://freshaquarium.about.com/cs/qa/f/faq0050.htm