The shock waves aren't good for your fish but I doubt that you killed them. If your waterfall is leaving a small hole open in the ice then that should be enough.

Lisa

you are making two mistakes.you should not run your waterfall in the winter,you could get a ice dam,that is a build up of ice from the water in the pond.it could lower the pond level too far.dont pound on the ice, it is like you being in a drum,.you could hurt your fish.try drilling a small hole in the ice and get a small aquarium air pump.put the pump in a small plastic tub,to keep it from the weather and run the air hose thru the hole in the ice.this should keep the pond open.i try not to disturb the fish and let them hibernate during the winter.

I leave my waterfall running all year. If you're running the waterfall, did it not keep a hole open?

I had a waterfall freeze over last year preventing the water from going back into my little pond, so it drained and I lost some babies.

This year my big pond froze solid when my bubbler stopped working. I quietly cut a new 1 inch hole with a cordless drill and left a fish tank heater in there for the day, which opened a large hole so I could drop a new bubbler there.

i use a pond de-icer that i bought on amazon,it has a thermestat that turns on when the temp.reaches 45 this keeps a hole in the ice which is what you must have in order for the co2 to escape.that will keep your fish alive with or without the falls running.if you keep your pumps running with the de-icer near the pumps they won't freeze if you don't keep the pump running it might freeze and will be ruined.i have to ask are the koi dead?if not don't worry about it but don't ever break the ice if you do not want to kill your fish.