do you have anything running other than the heater?an ice dam(built up of ice on the water fall)could cause that.that is too much water to loose over night.you could not add any water to the pond,and see where the water stops at the leak.if the leak is at the bottom dont let the pond drain down too far.

Hey there. A heater is much more than you need. You are wasting energy keeping the whole pond and the surrounding air warm rather than just keeping a small space open(just a sliver is enough) in the ice. The water is deep enough that the fish should do fine. Can you find out the water temp a foot or more deep? It doesn't need to be warm. My pond is 34oF a foot down with the ambient temps 20oF and with a bubbler and the waterfall from the Skippy filter going it will still be open if the temps drop much lower.

I tried to use a heater on a very small pond once and the surrounding shrubbery was totally white with frost and icicles and there was a fog completely surrounding the pond. Far too much heat.

Turn off the heater, turn on a bubbler. Let the waterfall run unless it forms a dam. The bubbler will likely be enough. If not, then get a deicer, not a heater. The cost to run will be much, much less. There is even a theory that just so there are bubbles from the aerator, you don't need a hole in the ice.

The fact that your fish are still active shows they are not settled for the winter yet. Check the water temps. They should slow down below 55oF.

Sorry, I'm too lazy to do the conversion to Celsius today.