How do i keep an airhole in my pond as it freezes?

My pond is an oval kind of shape being around 12ft long and 7ft wide,its deepest point is 4ft.the water is pumped up from the bottem to a large gravel filter and then it runs into a biofilter with uv light and cascades back into pond via 3tier waterfall.Last year the pond and waterfall froze solid so i had to switch the pump off,when it eventually thawed all my fish were dead,there was also around 50 dead frogs (big ones) Im guessing it was the frogs thad died first due to not being able to breath and that there carces poisened the water,does that sound likely? The pond has started to freeze already and the waterfall is all icycles,im sure there will be frogs in again though i never see them,how can i keep a breathing hole in the ice,is there anything that i can buy from internet or is there an old remedy at hand.Thanks in advance for any answers,regards skip.