Gave it a try. Got a decent bit of clayish topsoil, stuck it in a bucket with a hose, rinsed it out a bunch to remove fine particles and dirt, and mixed it into the first bit of sand. Ended up with just a few big handfuls, the small particles all rinsed out. Tank was somewhat cloudy on first filling, but a piece of felt in the filter and a 60% water change fixed that just fine. There was no bad smell or any signs of any other issues, so after adding some sticks and filter media from a fully cycled tank, I added a tiny baby bushfish. He's an anabatoid, so he's safe in lower-oxygen environments.
There was a very mild sulphur smell in the filter area about 5 days after setup. I did a big water change and waited a few days, and it's pretty much gone. The bushfish has shown absolutely no signs of distress, and he's not air-gulping any more than is normal. I don't think there's anything more in there than you find in the average muddy/leafy pond. The scent could potentially be from all the leaves I dumped in, but I'm reasonably certain that it was the clay.
Clay is mostly along the corners and sides, where I intend to plant things. A couple of val sprouts that I planted are already showing growth, so I'd say plants like the stuff. Using a whole bunch of clay soil probably wouldn't be immediately safe for fish, but a couple of handfuls is evidently just fine, at least for a fish who usually lives in ponds anyway.
Baby bush seems to be enjoying himself. He has lots of places to hide and nobody bothering him. Also, there are multiple species of tiny decomposers (itty-bitty worms, copepods, limpets, etc) all over the glass.
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My current project, a 65 gallon aquarium stocked with vernal pool fauna.