Find a disposable little cup: a carry-out "sauce" cup, minus the lid, is ideal. Or cut down a plastic cup to 1" or so. Fill it half full with vinegar (any kind) and add a bit of something sweet, then put it close to the problem area(s). The gnats will suicide happily. (Add more vinegar as it evaporates.)

These gnats like moisture and almost anything edible. Wet potting soil, split tomatoes on your counter (another name for them is "fruit fly") ... if all else fails, they will survive happily on the dampness in your sink drain.

Water using water treated with mosquito dunks.

Most effective using rain water or water pre-treated with something to remove the chlorine/chloramine (available at pet supply stores in the aquarium section).

Last year our office atrium 14x20' was re-potted with bagged soil. Soon after we had hundreds of gnats buzzing around at all times. We used a wide array of biological controls and pesticides and found that continually watering with mosquito dunk treated water was the only 100% effective solution.

Fungus gnats are different from fruit flies. Depending on the size of the bags, you could try putting them in a chest freezer for a day or two, or if you're using the medium in containers, pour boiling water through it before using it to kill the larvae, or water with Bt as suggested above. Difficult if you're dealing with a large quantity of medium I know, but doable in smaller quantities. HTH

Another among many reasons to dislike this awful product.

If the potting mix came with fungus gnats, it was *wet* when you opened the bag.

If dry, or moist, then you have applied too much water to a high-organic mix.
So not a problem with the mix, at all.

Meant to add --
If the mix was wet when you obtained it, then you should have returned it. Sometimes the stuff is stored outdoors and gets wet. Not a good thing.

I agree...it's not the mix, but the storage before or after you purchased it, or overwatering. There are certain places that I don't buy materials from in the spring, because they have stored stuff outside all winter; in the spring when it thaws it is soaking wet. So come later in the year when all of last year's stuff is sold, I'll buy from them, but not sooner. I've learned my lesson due to finding all the organisms in that wet stuff in my house or garage.

I think that it also the mix. It is comprised of a very finely chopped peat moss, a perfect breeding and feeding ground for fungus gnats. It is also a potting medium that tends to stay mucky for a long time, even if you try really hard not to over water.

Kind of hard not to get it too wet when sprouting seeds in it . The problem is with the MIX, as the gnats didnt come from the water LOL. And some are right this stuff was left over from last years store inventory and stored outside. I guess the trick is to buy a bag for next year and keep it in the house for use next spring
Yellow paper with petroleum jelly spread on works as a trap also.
I have been watering with mosquito dunk water .
Is a hydrogen peroxide/ water mix also effective ?

rhizo: My comment was kind of terse, but the soil deserves the bad comments it gets. Thanks for explaining why it is so bad. The water retention and fine structure make this a very bad soil to work with. It is too water retentive, attracts pests and diseases. Some people have made it work by watering in sips, but this causes other issues including salt build up. The best soils are the custom mixes made at home as most commercial mixes which are suboptimal. People have incorrectly learned to use and buy peat dominant mixes when they have better and cheaper options. Peat based mixes make gardening much harder then it really has to be. It's amazing how much better less experienced gardeners (and experienced ones) can grow when using a superior fast draining mix. The overwatering issue so common in peat based soils is no longer a factor in superior mixes such as bark based.

When using BT (mosquito dunk) water, make sure to remove chlorine / chloramine.

Also, this treatment can take up to 3 weeks due to the lifecycle of the gnats before they are all eliminated.