What it looks like
Small black flies around the pot, especially when you touch the soil. Translucent larvae in the substrate. The plant usually is not harmed, but the flies are a nuisance and multiply.
Why it happened
A permanently damp soil surface. Gnats lay eggs in wet topsoil — without it they cannot breed.
What to do
1. Let the top 3 cm of soil dry out properly between waterings. This alone fixes most cases. 2. Put up yellow sticky traps to catch the adults. 3. Add a 1 cm layer of sand or fine grit on the surface. 4. For heavy infestations, water with a Bacillus thuringiensis israelensis solution.
How to prevent it
Do not water on a schedule — water by the soil. A dry surface is the best prevention there is.