Fungus gnats require a medium that never dries out in order to reproduce.
If you can let the media get really dry between waterings, you should be able to get them under control.

Yellow sticky cards way down by the soil will also help. So will physical barriers ( ie a paper plate) preventing adults from burrowing into the media to breed.

Worst comes to worst- your hardware store/ Home Depot will have " pond donuts", which are used to prevent mosquitos from breeding in backyard pools. The active ingrediant is Bacillus Thurengensis ( isreali), or BTi. ( As opposed to Bacillus Thurengensis (kurstaki), which is the one commonly used to control catapillars on field crops.)
BT is a bacteria which produces a toxin. The action of this toxin is to radically alter the pH of the insects digestive tract. Basically, unless your digestive plumbing is based on an extremely alkaline envionment, BT is harmless. As a mammal, your digestion is acid-based, so it's non-toxic. If ya want, I'll eat some.

The differant strains of BT are effective against differant pests. For fungus gnats, you want BTi.

One of those pond donuts is about a billion times more than you need, so crumble a little bit off, dissolve it, and water it in. Two donuts in a package oughtta last you a lifetime. 'Bout $10 US. I don't know of a cheaper way to buy it.

I generally find the Miracle grow products to be a little too hot for young plants. I suspect that you're looking at a micro lockout due to either pH issues or macro overdose- give 'em a good flush, flush 'em again, and then let those poor girls dry out- their toes are getting all wrinkely!