Really sorry to her about your fish, particularly the 20 year old. There's something special about a fish of that age.

I can't give any advice about the fish. The reason for keeping a hole open in the ice is, as I understand it, to maintain the oxygen levels in the water, ie to allow some open water surface for gas exchange. The threat is that rotten vegetation in the bottom of the pond will release gases which have nowhere to escape.

So it doesn't depend on fish and frogs being able to locate the hole and stick their heads into it.

That said, I don't do anything with our ponds, and we don't notice any diminution of our frog population.

Therefore, I would deduce that the frogs would have died anyway.

I would also console myself with the thought that once you have frogs, numbers are not a problem (our highest count was 170) and you will always lose a few.