fungas Gnats in my soil, looking for cheap control methods

Mosquito bits. I put a teaspoon on top of my 1 gallon pots. After I transplant to the 5 gallon squat pots then I add another tablespoon to the top. I always put it on top of the soil of my 5 gallon pots but I only put it on the 1 gallon if I notice fungus gnats. It takes some time for them to show up so usually my plants aren't in the 1 gallon pots long enough for the gnats to show up. But they spend much more time in the 5 gallon pots. The label says it can also be put directly in the nutrient mix but I've never use it that way.