Answer: Based on your description it sounds like there may be fungus gnats living in the potting soil or possibly in the drainage saucer under the plants. These breed in wet areas, so to stop them you need to modify your watering practices.
First, always empty the drainage saucer under the plants after watering to prevent a constantly wet environment. This will also benefit your plants in that they do not tolerate overly wet soil.
Back off on the frequency of watering also so that the soil surface dries out between waterings. (This will also benefit your plants as they do not need a constantly sopping wet/saturated soil.)
As an additional deterrent, you could place a thin layer of fine grit, gravel or sand on top of the potting mix. This fast draining layer will dry out almost immediately after watering, thus preventing the fungus gnats from breeding in the soil.
The above steps will prevent breeding and stop the fungus gnats from reproducing -- they are shortlived and the adults will die out in a week or two. If you want to stop the adults immediately, use a soil drench product labeled for fungus gnats such as Gnatrol.
I hope this helps!
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