Build a synthetic wall equivalent to the height of an aquarium tank. Make it so that the wall appears to be made out of stone and covers the width of a moderately large underwater space as seen in aquariums.
Consider cheaper materials in making the wall, primarily because it'll be hidden behind faux underwater vegetation later.
Cut an opening in the top midsection of the wall that's wide enough to let water out and where the illusion of the waterfall will come from. The waterfall can vary in width.
Cut another opening of same proportions directly below on the bottom face of the wall. Know that this acts as a reservoir to catch fine sand into a circular current through an inside water column.
Have a professional prop designer cut out the vertical water flow column inside the wall. Ask to have it cut approximately six inches from the bottom of the wall on through to the top for a small opening that later balances the internal current.
Set the wall up along the back wall of an empty aquarium tank. Face the wall openings forward, toward the viewer's perspective.
Find a long piece of elastic tubing. Be sure it's lengthy enough to reach from the bottom opening of the wall column up to the top back of the wall and where the water line will be.
Make sure the tube has no clogs and is able to make water and fine sand flow through at a steady rate.
Place an air stone at the bottom opening of the wall's water column. This allows bubbles to float up the column and to the main opening in the front of the wall.
Fill the aquarium tank with water.
Pour fine sand into the tube opening at top of the water line and tank. Make sure the sand travels all the way through the tube where it'll mix with the air stone bubbles to create an upward current.
Check to make sure the fine sand is coming through the opening in the front of the wall. Look to see if the opening of the water column at top of the wall is creating enough strong current underwater to pull the sand and air bubbles upward.
Make sure the current is pulling the fine sand up and through the wall opening at a slow rate and not a fast rate.
Place a sloped container near the reservoir at bottom front of wall to recapture the falling sand outside. This sends the sand back up the internal water column and out the wall opening again for a circular pattern.
View again from front of the tank to make sure the circular mix of water, air stone current and fine sand creates the convincing illusion of a gently falling underwater waterfall.
Obtain faux vegetation later and place around the wall to give the illusion of a waterfall in a fantasy underwater forest setting.