I use the perlite method in all my soil containers.
IMO- it's works better than any other product to keep unwanted bugs out of your soil.
and help to keep moisture in your soil.
It works better than the diatomaceous earth.
You'll kill all the bugs (Good or Bad) in your soil if your using oil based bug products.
Something to keep in mind.
The other thing is maybe the fungus gnats are coming from some place else.
Like your drains.
You get rid of them in the soil, but they might keep coming back to your soil.
Migrating from someplace else in the house.
I agree with
charface make sure you know what bug your dealing with.
The fungus gnats are small. they look like fruit flys
They like to crawl around on the top of the soil
or hang out by your drain hole.
I also agree with
WillieMazeHaze.
You might be giving your soil to much water.
something to keep in mind
I'm sure you'll be okay. your on top of things. it's good to try and catch a problem quickly.
Your doing a good Job