@ highlighter- The average fruit fly trap is not the most effective way of getting rid of the fungus gnats. These bugs can number in the hundred, and most traps do not effectively target the larvae. Here is a guaranteed fix that will rid your plants of fungus gnats for good.
1) Cover the soil in your houseplants with a quarter inch fine cleaned sand. Insert sticky sticks in the soil around the plant (like sticky flypaper traps, just sticks).
2) Water the plants with the organic insecticide BT (made from chrysanthemums).
The sand will dry out the top layer of the soil, forcing the flies to burrow to the surface and look for a moist place to land. This moist place is provided by the sticky sticks. BT is a larvae killer, and will kill the larvae of almost any bug. After a couple of weeks of this treatment, repot the plant and choose a faster soil medium. Also, be sure to keep the plant soil free of dead leaves, and exposed roots, which will attract these annoying pests.