looks to me like you'll need a reservoir and a lift tube with elbows. if you want the air and sand out of the same hole you'll only need 1 elbow. if you want them separated you'll need 2 at least.
very interesting, i'd like to hear from someone who's made one.
I'm actually going to be tinkering around with one of these myself. No clue if this will work but this is what I have so far. As a rough draft prototype model, I have 1 1/2" pvc piping. Might actually be too big but I already had it laying around. The pipe runs out of the water to allow the air bubbles to escape (I'll tinker around with this more later). About 2" below the water line, the pipe has an opening cut out and a fabricated slide (more pvc) sitting at roughly a 45 angle. This should allow the sand to fall back down, hit the slide and flow out. At the bottom will be a constructed pool of some type. I got a small plastic reptile food bowl for this purpose but may give way to a 2" or so pvc pipe. This will be connected to the main tube to allow the sand to flow into it where the airstone should carry it back up to the top. I have not purchased the pump or valve as yet so not sure what size I will need. I'm currently thinking a pump for a 20g tank should be fine.
If this works, then the hard part of trying to build landscape around everything will follow. This will be difficult (for me at least) as I'm not sure how to work around that. My goal is to get a functioning model first. I can worry about decor later.
Haven't been able to work on it lately though. Been sitting under the stand collecting dust waiting for me to get some time back in my day. If it works, I'll surely post up pics of it. Maybe then some can advise how to create the landscape around it.
I do know for the upper tray (where the sand comes out of the fall), I want a ridge of weeping moss.
Good luck on your project and let us know how it goes.
I built one of these a while back. I used 3/4 in pvc piping with .5 inch holes drilled about 2-3 inches from the bottom of the tube and about half way up the tube. I then used an elongated funnel purchased at autozone as the reservoir. I cut it at the the bottom so it snuggly fit the pvc just underneath the hole drilled at the bottom. This was all sealed together using aquarium sealent from Home Depot. I then built a plexiglass entrappment for the device so I could create a customized natural look. Check out the clip of it in action.
I no longer am using this due to the unsightly airbubbles streaming from the center hole but recently found a way to solve this problem and will be building another shortly.