Safer Yard & Garden Insecticide, Safer Garden Fungicide can be found at most garden centers. I like Hot pepper wax spray.

For gnats the only thing I have found that works 100% of the time is gnatrol. It's hard to find so if you do buy the large quality.

Have you tried using a 30x microscope and see if there are any other signs of bugs? Normally a magnifying glass is fine but trips can be very small.