a zoo in canada one had to be euthanised when it slipped on its frozen pool. you wonder, what on earth was an aquatic species like a hippo, which is a warm temperate animal at best being put outside in the snow with a frozen pool. another was kept in an unheated wooden barn for over a year i think and that caused a stirr too (may have been the same zoo again) and teh zoo was taken to court.
I think the zoo- folk in these very cold climate countries just presume that because they are used to that climate, tropical animals can get used to it too. I don't much like seeing photos of tropical animals against 'polar' backgrounds- I once saw a photo of a Gorilla in Snow, again in a Canadian zoo. It seems all wrong to me.
With Hippos, I think a basic requirement is they should have access to heated pools at all times, but in colder countries, that's a very expensive, almost impossible, undertaking. This is a species which seems to suffer from poor housing generally in our zoos over here. Even Whipsnade's, which are probably the best exhibit, have a large unheated outdoor pool which is usually filthy and full of dung- again, an expensive operation to empty or clean it or provide a filtration system.