What's cooler than an underwater waterfall? How about two?
The plants in this tank are a bit overgrown and probably due for pruning unless the aquarist is going for the wild growth look. This trivial commentary aside, the dual cascades over seiryu rock are some of the most spectacular, dramatic, and realistic we've seen.
An Underwater Mirage
Underwater "waterfalls" are really sandfalls to create an illusion of whitewater breaking over rocks. An air pump is used to carry very fine white sand (usually silica sand) up a tube where the grains cascades back down through the aquascape into a collection basin.
It takes talent to create a sandfall that naturally mimics waterfalls - even more so when it is a multi-tiered cascade. Careful planning is also required to make sure the grains of sand all fall back into the collection basin. Otherwise, the sand will eventually build up where you may not want it to. Take for example this beautiful aquarium suffering from the design flaw where sand is pooling at the bottom front of the tank:
Want to try this yourself? Here is a step-by-step video about how to create an underwater waterfall: