Then you build your waterfall basin.
You need a submersible pump (smaller is best), a riser(a tube to bring the water up)
A reservoir; ie a container that is significantly larger than your pump, but small enough to fit inside and shallow enough that you can cover the edges with whatever substrate rocks and other hard scape you pick.
You need a relatively substantial cover support for your waterfall basin, that will take the weight of your rocks, but is open enough to let the water through. I ended up putting a wire plant stand over a frozen eclair bin, but you can see some of the other options I tried, such as PVC unions, an upside down coair basket hanger, upside down terra cottage pots or, if you have enough, pieces of them will all work. Only your imagination (and water flowing downhill) limits you.
You'll notice I have one big rock that is the primary waterfall feature. If you want a waterfall this is mandatory and dangerous... when you test your layout, be Absolutely sure you have some sort of padding to protect the glass on three sides, including the bottom. , I used tree bark, because if it tips and hits glass, it can easily shatter the tank