I've come across a lot of "worm" threads lately, and even had a miniature outbreak of my own. the worms I had were small - no tiny - about the width of a hair, maybe 1/16" - 1/8" long. Is there any decomposing/dying/dead plant matter or other detritus in the tank? I saw something about a "detritus worm" that I posted on someone else's thread, a brown, segmented worm that was just plain nasty...Although generally non-problematic, but a large outbreak is at the very least unsightly. Frequent large water changes are no doubt helpful and probably your best bet at keeping the population in check.