A waterfall wouldn't be that hard to do no expensive. Use the same thing we are doing with the PVC
pipe. Cut a length of pipe in half that fits your needs and use a small section of tubing that flows out of the tank onto the waterfall.
I would do it this way:
Take a 3 inch pvc
cap and cut a small section out on one side that feeds that water fall. Take a 2 inch pvc
pipe and cut in half. Catt the pvc
pipe with silicone and gravel. silicone the end cap and pipe half together so that the pipe is below the cut out. Take a piece of tubing and heat it up so it will hold the correct U shape and cool. Place the U shape completely in the water to fill it and hold the ends. Put the ends in the cap, that should be full of water.
It is really hard to explain but make the tubing just long enough to keep water flowing but to prevent the water form overflowing in the 10 gallon if the pump to the 20 from the 10 fails.
I imagine there is a way to either build up the sides of the tank tgo suppoert a waterfall becaus eI wouldn't want to etch out a section of my 20 gallon to make a waterfall, even though that is always an option. A dremel tool with a fine sanding disk or grinder will eventually cut an overflow that could work. All depends on what you want to do.