It's the soil.

If they store it where it can get wet, this is the result.

When the soil stays wet with no plants in it to draw out the water, the organic material begins to break down and rot.
The anaerobic rot is what lowers the PH and attracts the fungus gnats.
(Also attracts root aphids, they are much worse!)
They lay eggs in the soil and their babies eat the fungus that the wet conditions encourage.
If they are already hatching, it means you are very heavily infested.

How to fix it?
Tall order.
You can try baking the soil in an oven.
(It's gonna stink).
Bake it moist, 400 F. until it is bone dry.
Then add some finely ground dolomite limestone and do some serious hoping.

What would I do?
Dump the whole mess, scrub everything with bleach, buy new soil from a different shop, and start over.

And here a little tip.
Add a tablespoon of Adams flea and tick shampoo per gallon to the first few waterings and you won't see anymore fungus gnats, or root aphids.


Good luck brah, you're gonna need some.

Weezard