The only way you'll be able to get that to work is by removing the excess water between the sand particles, ignore that step and you'll discover you've created an environment that won't allow roots to function normally.

Fungus gnats prefer any areas that combine constant moisture and organic matter that breaks down quickly.

Al

I'm afraid I don't follow. What is "excess" water?

The major drawback of sand as a growing medium is how rapidly it lets water drain. Soil doesn't ordinarily produce gnats, although it drains more slowly. Gravel must drain even faster than sand, but it does get gnats. I think that must be because gravel has damp spaces between the particles where gnats can breed, and soil doesn't. Sand doesn't either.

So... I don't follow the logic of your comment, and assuming it's true, I don't see what action it indicates. Sorry, I appreciate your willingness to help, but your advice isn't clear.

"Sand" covers a lot of territory. What comes to most people's mind when the word 'sand' is used is the stuff sand boxes are filled with. Even torpedo or builder's sand would be difficult to grow in. Here's why:

How much water sand can hold is directly related to the size of the particles it's made from. Fine sand, like 'play sand' is very fine and will hold a very significant amount of water between the particles at the bottom of the pot; >6", I would expect. This occurs when the sum of the forces of adhesion (water sticks to soil particles) + cohesion (water sticks to itself) are greater than the force of gravity, causing water to 'perch' above the pot's bottom. Horticultural sand the size of BBs is incapable of holding perched (excess) water between the particles because the significant reduction in collective surface area reduces the force of adhesion so greatly it's gravitational flow potential alone forces it to leave the pot.

If a medium supports 6" of perched water, and the pot you've filled with it is 6" or less in depth, the medium will be 100% saturated at container capacity (o/a water retention at the immediate point in time when the medium has stopped draining after being fully saturated.

Company here. Perhaps someone else will pick up the ball. It's all explained fully below:

https://www.houzz.com/discussions/6167237/container-soils-water-movement-and-retention-8-26-21#n=6

Al

Assuming you mean fungus gnats, which are the kind most plaguing to houseplants, they require organic matter to survive....or at least for their larvae to survive (adults don't really eat). Therefore, adult gnats are highly unlikely to lay their eggs on gravel or sand, as instinctively they know it is not a hospitable environment for their young. In fact, topdressing a container planting with coarse sand or gravel is one of the recommended measures to control fungus gnats, as the adults will not lay their eggs. If there is little to no organic matter in the gravel or sand, any larvae that might exist won't live long. It has really nothing to do with any wet or damp voids within the mix. It is consistent dampness and available OM.

btw, those open voids or pore spaces between the media particles are absolutely essential for aeration and proper root development. If they are inadequate or too small, your plants will suffer.

Why don't you try the coarse sand, and see how it goes. If it works out and you have no gnats, maybe you can use the sand repeatedly without having to wash it. The gnats don't like sand, so they may avoid it completely. Years ago I would buy the cat grass seeds and grow them. It would take about a month or more for them to grow, and the cats ate all of it at once, so I decided it was too much work and time, and I'd have to grow many pots to give them grass daily. I grow my cat grasses outside now, and there's always more than enough. Only in winter do the cats go without it, just like all the wild cats go without greens in winter. My cats are indoor cats. I've never tried washing coarse sand, but it sounds like a pain. Sifting, hand picking, immersing. I have a lot of sand loving plants, so I would just dump the old sand in one of those beds. Coarse sand may be bad for root growth, but grass doesn't do well in pots anyway. I don't think that I ever tried seeing if I the eaten cat grass would regrow. Grasses have enormous root systems, so aquarium gravel would normally drain easily, but when it's totally inundated with grass roots, we'll that's Living Excess OM. If all else fails you can try mosquito dunks. I was wondering how to improve drainage in pots using only sand, in a pinch.

I started to watch this vlog about using sand, but didn't have time to finish. I thought of it when I read your query. Not sure if it answers the question of how to recycle it, but might be worth watching.

https://www.youtube.com/watch?v=QsZHSkG5xsE

BTi will get rid of the gnats.