I found out a way to make a smaller one, though I haven't used sand yet.
I took a water bottle without the cap and put two holes on the side near the bottom and up near the top. Then I put it in an aquarium with an airstone (I think it's for 10 gallons) and watched to see if the bottom would suck up an debree and it did!
The top of the water bottle was out of the water and the debree shot out of the side hole near the top. Haha
So that amount of space should work to make a sand fountain/underwater waterfall.
"Life is an adventure full of beautiful things, and a treasure worth more than gold."