I'll ask again - when the waterfall is off and the water all flows back into the pond, do you still need to add water?
I know It seems elementary, but years ago we had a similar question. The poster thought the pond was leaking because the water level fell when the waterfall was on - however, when it was off the water level returned to normal. He wasn't taking into consideration the amount of water in transit. Every single new pond owner's first thought it "I think I have a leak!"
So it bears asking - how much water do you lose when the falls are running and does the water level return to a more normal level when the falls are off?
Also - where are you located that you were able to just finish a pond? What's the weather like?