Sounds like a midge. If so...... your screwed. I've had them for about a year and nothing will get rid of them. I used a product called microbe lift and they seemed to be gone, for about 2 weeks that is. I posted on here about them several times and still couldn't find anything that works.
You'll know if they are midges if you start seeing 1/4 inch long, brown tubes/worms attached to a bunch of stuff in the tank.