http://www.planetnatural.com/pest-problem-solver/houseplant-pests/fungus-gnat-control/

This might help you

Keith

The BTi is your best bet, you'll need to be consistent and use it every time you add water to your terrarium plants for a while (minimum approx. 3 weeks) but it will stop the larvae from developing into breeding adults, stop the breeding cycle. It absolutely works. It works at the larval stage, you really don't need to concern yourself with the flying adults already present - they live only a few days and if the breeding cycle is interrupted, they'll be gone with nothing to replace them.

It's a naturally occurring bacteria that is of no harm to anything but mosquito and gnat larvae - it is not a synthetic pesticide. You wouldn't have pets or birds, fish in your enclosed container but they would be safe with the BTi as would you ;)

I've ordered this product from this particular vendor and can recommend them. When looking for the link for you I was very surprised to see the current price. The product has a shelf life of about 2 years so while there, I just placed an order for myself to take advantage of the savings. I grow a lot from seed (continuously moist seed flats/pots) and can be occasionally troubled by the gnats, I try to keep the BTi on hand.

Here is a link that might be useful: Gnatrol WDG (Bti)

You can set a trap for them that can help. A saucer with sugar-water and boric acid attracts them and kills them. Also I've heard that a saucer with wine will get them drunk and they'll drown in it. Although if they're breeding in the dirt it can only do so much.

Bti can also be obtained from pretty much any home improvement store. They are sold as "mosquito dunks" -- crumbly "donuts" that one would put in a rain barrel or pond. Simply put a quarter of a "donut" in a gallon of the water you will be using. Let it soak for a few days then pour off what you need to water the terr and even your other plants. Leave the chunk in the jug and refill. The chunk eventually disintegrates, at which time you add a new chunk.

You can also try carnivorous plants like Drosera (sundews) or terrestrial Utricularia (bladderworts). Most species do seem to require pretty strong light, however.

I used fenbendazole to eliminate the problem of killing off the eggs and larvae. Has worked like a charm, has not harmed the mosses, lichens or plants and have not seen a single gnat or any other bug since I started using it. Purchased it as a dog de-wormer and mixed a small amount with water (shake to mix VIGOROUSLY). I used a dropper as opposed to spraying with it to get it deep into the soil and down into the layers of pebble and charcoal as well.