m0useanswered grow question 7 months ago To deal with gnats in a growing environment its best to use BTi or Nematodes. they will kill them all in a week or two. repeat applications maybe needed in a few weeks but its cheep and effective, Sand can work if its an even later across the medium but all the added extra weight of it compresses the substrate.
Your plants do look a bit small. maybe they need more food, best to start a diary and link it to the questions you ask so others can have a better understanding of whats happening.
Good Luck!