Making a waterfall within an aquarium is simply creating a small tabletop fountain. You will need the water to fall from a height above the surface of the aquarium water, so consider the design of your tank and the point from which you want the water to fall. A rock precipice (real or fake) is the most realistic origin for your waterfall, but with a little imagination, you can create endless interesting paths for your aquarium waterfall. The first step is setting up the waterfall pump.
Purchase a submersible fountain pump small enough to fit in the aquarium, with a low gallons-per-hour (GPH) rating. Three brands that make small submersible fountain pumps are Rio, ViaAqua and Fountain Pro. In 2009, these cost from £5 to £8. For the small space of most aquariums, a 40-GPH pump is sufficient.
Purchase flexible vinyl tubing that matches the size of the pump outlet. Connect one end of your tubing to the pump outlet.
Cover the inlet of the pump with screening to catch droppings or food leftovers the fish will leave in the water. You will need to clean this off periodically to maintain pump strength. Consider creating a screen "box" to place around the pump that will hold clogging materials away from the pump.
Place the submersible pump at the bottom or downhill side of the waterfall. The pump must be fully submersed in water because it is water cooled.
Run the tubing to the top of your waterfall. Create visual interest with the water spilling over a rock ledge rather than the bare end of the tube. Add water and turn on the pump to enjoy a recirculating, closed-system waterfall.
You can create a waterfall using any nonporous, nontoxic material such as plastic, glass or ceramic as well as rock.
If your outlet tubing is smaller than the pump outlet, the pump motor will wear out faster than normal. Be sure to match the diameter.