Fungus gnats are a difficult infestation to be rid of and easy to get since they can come in a lot of potting soils. You pretty much have to pull the plant out of the enclosure and let it dry out completely. If I remember right, the eggs hatch about every two weeks.
There are treatments, but it would leave your cham without a plant for a while. Researching on house plant sites and forums can help a lot with learning to control the pests.