I saw these same little buggers in a 10gal a friend of mine gave up on. He tried everything to keep fish alive in it and I don't think he knew about the cycle etc. Anyway, when he asked if I wanted it and I said sure, I'd take another tank I went over to break it down for him. There were hundreds of these little larve, not to mention the gnats, slightly bigger than a fruit fly and lime green. I never did find out what they were. I checked the tetra whisper filter he had on the back and the cartridge was absolutely crawling. Also the gravel was full of em once I got the water drained. Your problem may not be confined to the filter I'm afraid. Hope someone can chime in with a species or soultion. My solution was to let the tank sit outside till the water dried up, dispose of the gravel and filter cartridge and bleach the heck out of everything else. Keep in mind this was a full on infestation in a 10 gal. I can't imagine the potential for a larger tank.
