I had the same problem but the flys that were reproducing in my tank were diffrent. There would be like aton of dead ones on the water surface. They would reproduce and I would see little larva(they look like little red worms) swimming in my tank. There were also little hollow tubes that these worms lived in. I solved my bug problem by sealing all openings were they could get in, especiall the filters which I think they use as a place to reproduce. Now I hardly see these guys around my tank anymore.

or lizards {{{{gecko's}}}}