First, a bit of background story. I've been experimenting with various methods of propagating succulents. One method involved putting a couple sheets of paper towel onto a tray of pebbles. Misting the paper towel until it was damp. Then putting the leaves on it, once they started growing roots. This method worked very well, and I have some really nice succulents to show for it. However, once the succulents were ready to be moved to soil, I disliked having to separate the plants after they established roots in the moist paper towel.

After most had been transplanted to soil, I decided to continue with a similar method. Instead of damp paper towel, I put a thin layer of sand on top of some pebbles. Now when the plants are ready to be transplanted I can pick them up without damaging the roots.

The problem is, at some point while the first plants were growing with the damp paper towel, I started to notice very tiny black bugs. I've tried to identify them online, but nothing seems to describe what I see. They aren't fungus gnats, because there is no soil, and they don't turn into flies. I thought they might be black aphids, but they are very rarely on the actual plants. They are almost always under the plants, or crawling on the sand. They don't cluster together the way aphids do, and they seem much smaller (you can see in the photos they are smaller than the grains of sand)

Does anyone recognize them? At this point, I am prepared to let the sand dry out, or pour some rubbing alcohol onto the sand. I don't mind losing the plants if it means getting rid of the bugs. My concern is that the first few batches of succulents, that are now growing in soil, will have the bugs too.

I've tried very hard to see if I can find them in the soil of any of my other plants, but it would be nearly impossible to see them on a dark surface.

Side note: I had boiled the sand, and pebbles before putting any plants or paper towel on them. The leaves all sat in a plastic container for 1-2 weeks until roots started to grow before I placed them onto the moist surface.

All help & advice is greatly appreciated!