I have had the same type issues plants are okay, but not Topps.  I got an infested bag of starter mix. Fungus gnats.  But I'm seal team 6 when it come to those LITTLE  maggots  BTI aka. mosquito bits, Sand on top, neem cake and diatomaceous earth mixed in and oil on top, bottom water, and damn sure get a clean supply of everything. Nemotodes are my back up. And I just swaped the peat for coir. Some chips & some pith. They come compressed in bricks, and I rehydrate w BTI treated water. Hopefully if I don't bring in an infestation it will be the last of them. But I feel like it's gonna be cat/mouse. We will see.