I did this in my 72 gallon bow for my Fire Belly Toads when they ruled the tank (since converted back to a full aquatic set up for the ACF).
I am awful at describing the steps so if it is confusing just ask me and I will clarify what I did lol...it was a trial and error thing for me.
1) I have a canister filter and I had the intake below the water line on the right hand side and I the outflow for the water fall aspect on the left hand side (for your visual purposes).
2) I took a large plastic critter keeper tank and submersed it in water then turned it upside down on the left hand side of the tank (to be under the outflow). (Figured it would weigh less than stacking a huge amount of rocks/sand).
I then built up the sand to cover the critter keeper and it made a nice sloping into the water "pond" area of my tank. (the right hand side with the intake for the filter)
3) Now, for the waterfall part, I needed to make a channel for the water that wouldn't erode away. (like sand)
I cut off the ends of a plastic soda bottle then cut it in half (so I had a long half tube), lined it with a plastic freezer bag that I used rocks and substrate to hide the extra bag on both sides of the tube (Think of like a pond liner with the bottle as the pond shell).
I used the two halves of the bottle (Trimmed to make fit my needs) to make it the length I wanted to reach the "pond area".
I surrounded both sides of the tubing with sand and rocks like a river bed and it helped camoflague the soda bottle/plastic bag area.
With in the tube I used a series of rocks to hold down / hide the bottle/bag channel.
4) In the "pond" area I had a lot of aquatic plants in the water column (like amazon swords etc), in the back of the pond area I had pots of pothos plants that grew through out the tank - the stems would suspend themselves in the water for the frogs to climb on or hold on to, and I had a large piece of driftwood so they could climb/perch when ever they wanted.
This was the end result -
The pictures are not a tutorial but I figured it could help . (I also had guppies and neons for a while in the water column - they had about 20-25 gallons of water or so and lots of live plants)
Frogs in the waterfall stream, again I had rocks around so they could either sit fully in the water or partially in the water.
End of the waterfall channel where it poured into the "pond" (I used some larger rocks as I didn't want a strong current pushing the frogs around when they wanted to sit in the stream)
Pond area
