I had an infestation of these critters in one of my tanks a couple of years back. I popped one under the microscope, and produced the sketch you see below. There was no observable effect on the fish, however. They were mostly around the waters edge, although I did see some on the water surface. I got rid of them eventually by crushing a few with my finger, everytime I went near the tank. I have no idea where they came from.
They're quite small (0.5mm), they jump, and are a pale tan color.
