Answer: It sounds like you may have brought in some fungus gnats. They breed in moist soil, so one way to stop them is to avoid overwatering the plants and allow the top surface of the soil to dry out completely. You could also put a half inch layer of sand or gravel over top of the soil. This works well if you have a plant that needs a constantly moist soil, because the gritty layer will drain and dry out quickly while the soil stays moist. Another solution is to apply a product called Gnatrol to the soil according to the label instructions. Whatever you do, it will take a week or two to see results because you have to wait for the current generation(s) to mature and die off. Luckily they don't live for very long and they don't do harm except to plant roots.
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