A sand fountain is also known as an underwater waterfall, since it gives the illusion of water falling within the aquarium rather than sand. An artificial wall is constructed in front of the aquarium's back wall and a plastic tube is placed within the gap. Air pressure supplied to the bottom of the tube makes the sand travel up the length of the tube along with the water current, fall down the artificial wall's surface and re-enter the tube through an opening at the bottom of the wall. This simple circulation of sand allows for a waterfall effect when looked at from the outside.
Empty the aquarium. Build the artificial wall for the waterfall as high as the aquarium itself using either a slab of ceramic or by purchasing artificial rock. The wall should be wide enough to hide the plastic hose and air stone.
Make a horizontal cut in the wall's midsection, 2 to 3 inches from the top, for the sand to go through. Make another cut 2 to 3 inches from the bottom for the sand to re-enter the tube. Make this cut inclined so that the sand slips in. Prop the wall against the back wall of the aquarium leaving only so much gap that the hose fits within the length. The minimal space forces the sand to move up the tube rather than settle below it.
Place a tube within the gap. A plastic hose is preferable to a PVC pipe since it is flexible and the placement angles can be manoeuvred. It should be as long as the wall itself. Make openings within the tube exactly where it meets the wall's opening, that is, 2 to 3 inches from the top and the bottom.
Submerse the 40 GPH pump fully in the water and place it behind the wall and exposed to the bottom of the tube. Place the airstone between the pump and the tube's lower opening so that bubbles are formed once the circulation starts. Place an inclined container in front of the wall's bottom opening so that it may catch the sand and send it back inside.
Fill the aquarium with water. From the top of the tube, pour down fine sand and observe the circulation. The sand should move out of the wall's upper opening and travel down toward the bottom opening of the wall. Bubbles should now be forming at the bottom of the tube which will carry the sand upward within it. The sand needs to be fine enough to move up with the air pressure but not so fine as to be blown around the aquarium as it comes down the waterfall. In order to have the perfect waterfall illusion, experiment with different sizes of sand. Finally cover the waterfall with artificial underwater plants to let it blend into the environment.