How odd that I saw this post yesterday then then last night, noticed about 4 white worms swimming in my tank. I don't have a picture, but they're really thin, about a hair width or smaller, white, and swim quite a bit. I've not seen them in the substrate. My rasboras and panda cory's have not seen the ones I've watched and hence have not eaten them.
Looking at the web for the types listed above, they might be detritus worms.