Hi,HH
Here's my procedure for permanently getting rid of soil gnats...using Mosquito Dunks
Just drop half a dunk in a 5 gallon bucket of your plant water,and 24 hours later, the water will be nicely populated with BTI- when you water your plants with it, the larvae in the soil ingest the BTI,and soon they're all dead (the larvae,not the plants)
Using this method,I haven't seen a soil gnat in my grow space for months- the Black Magic soil I use comes pre-charged with gnats,so I had a few when I started my current grow,but by the second watering,they were all gone-and they don't come back, because you're innoculating the soil every time you water your plants...
I'm not sure why more folks don't do it this way-it's harmless to your plants,and it doesn't kill the soil microbes like h2o2 can, if you use too much.
anyway,there's my 2 cents on the soil gnat thing...