Garden Frog In frozen pond

[RHD]

I have a half barrel water feature with a couple of water lilies and a fountain. The half barrel is about 3' wide by 18 to 20 " deep. In this three large frogs have decided to live. At first I couldn't figure out how they got in but then one day I saw one hop out and then a little while later, hop back up the side again from the grass. Those guys really can jump.
Last spring, after much croaking and splashing about, there were two blobs of spawn and they all hatched but very few tadpoles grew legs and made it out. I think maybe the big frogs ate them as I never saw any corpses.

With the unusually cold weather for Central London, UK, I have been worried the big frogs are running short of oxygen in such a small amount of water as I have seen one up at the surface trapped under the ice which is quite thick. This is unusual behaviour as they don't usually surface at this time they hibernate at the bottom and I don't see them for another month or so. I leave the little fountain off over winter it is not up to prolonged use so I only have it on a few hours a day when I'm in the garden in the summer.

I melted a couple of holes five or six inches across in the ice with hot water (keeping it well away from the frog) as I have heard breaking the ice by impact can cause shock waves which can kill fish. I'm glad now I did it this way after reading what you said about the ice keeping the temperature up.

Now I have made a sort of floating device out of tin foil and a tea light. The idea is that the tea light heats up the tin foil and melts a hole in the ice hopefully leaving a small hole to allow some oxygen in but 95% at least of the surface will remain frozen. I hope this strikes a medium between the ice keeping the frogs warm and the hole giving them enough oxygen. I'll let you know how it goes.