I'd say they're about the size of Lincolns eyeball on the penny. I'll try to get some pictures asap. And after looking into the two creatures you suggested they may be, you seem to be right.
I need some assistance... I recently bought some aquarium plants, and with them came detritus worms (which I expected) but after a while, small flea looking bugs started swimming around my aquarium. There seem to be two different types as well. Some tend to glide around, while others dart super...