WHY DOES THIS HAPPEN
From time to time you may notice small, annoying flies buzzing around your plants or crawling around in the soil. They're around the size of fruit flies, but look a little different - these are most likely Fungus Gnats.
These flies feed on various types of fungi and / or decaying plant material, and they typically show up shortly after watering as they like damp, humid conditions.
Unlike other pest infestations, however, Fungus Gnats don't tend to cause any harm to your plants, it can just be a bit irritating to have them hovering around!
SYMPTOMS OF FUNGUS GNATS
As mentioned above, Fungus Gnats aren't usually harmful to plants, they're more of an annoyance. However, in some rare cases, your plant may show some symptoms if there are a lot of Fungus Gnats in the soil, including:
Yellowing leaves (it should be noted that this is usually predominantly caused by something else - be sure to eliminate other potential problems, such as overwatering, before concluding that this symptom is a result of Fungus Gnats)
Damage to the plant roots - although the flies themselves don't tend to cause any harm, their larvae feed on plant roots. This means that smaller / young plants may potentially be affected