I was just outside an hour ago digging up some flower and transplanting them. I finished and I had a a pile of moist soil/clay/dirt/sand sitting in front of me. I grabbed some of it and and worked it into a ball. As I was looking at the ball it kind of reminded me of a huge piece of ADA aquasoil. I started making smaller pellets down to around half to quarter of an inch in diameter. I then broke those into smaller balls so they had a diameter of around 4-8mm. I put them in a bowl a misted it with water and spun the bowl around for a few minutes so they would get very spherical. They actually really started to look like aquasoil! I don't know if they have the same ingredients or anything. I also don't know if they will hold together correctly. I have about 2 dozen pellets in a cup with water and I will watch them for a few days. I will also try to figure out a way to produce them quicker and in larger quantities. I might just be wasting my just be wasting my time, but I will give it a try as an experiment. I probably never have enough to actually use. Let me know what you think about it, and if you have any tips as to the moisture of the soil while making them.
I have to think that once they've soaked in water it's going to turn into mud. If you have some way to fire them it might hold together though, who knows? At the very least, you've got the beginnings of a mud tank.
If you don't want to cap this with something like pool filter sand, you do need to bake the particles, or bake them, then break them up. I don't know what would be the appropriate temperature, but my guess is it would be 400F or higher. That's going to be a lot of work for enough substrate for anything except a nano tank.