Kelly,

What is the width of the waterfall, how high is it and how big is the pond? All of this will take in effect when sizing the correct pump. 5hp will cost you a lot in electricity.

One of our customers is using a 7500 gph pump (1hp) and his electrical cost is $80/mo.

Rule of thumb is 1500 GPH per foot of width of stream. This will give you a good flow and pleasing display.