tck:
Had same small fly problem you described. Flies would come out of the stool drain as it flushed. These were VERY tiny and just a bit bigger in appearance than fruit flies. ANYWAY....after trying ALL solutions mentioned here, with NO success, we finally ended up buying a simple spray can of Raid brand "Flying Insect " killer, and...after dumping black tank, and flushing as usual, and after adding our usual amount of water to the tank (4-5 gal.), I then opened the flush gate between toilet bowl and drain down to tank, and holding the spary/fogger can close to the inlet into the drain I sprayed (fogged) 3-4 times down the drain at about 2-3 seconds each time, then closed the gate. To preclude possibility that the fog/spray might damage the gate and/or drain seals or plastic, I then flushed the toilet through it's usual three flush cycles, and......WE HAVEN'T HAD A FLY SINCE!!! It's been about a month now, so it seems to have made a BIG difference. Steve & Lynette