Fungus Gnats

One year I tried vinegar and oil traps, layer of sand, diatomaceous earth, sticky tape. Sticky tape was probably the most effective of those for keeping numbers down but didnt get rid and was messy. It's a pain but the organic method that worked for me was a layer of grit over the top of every pot, tray or module in the area to stop them laying eggs in the first place

That's what I do too. And stand the seed trays/pots on a layer of grit because they get under the bottom of the pots, pernicious little critters.