Fish live in a totally different environment than we do. We create an artificial environment for them to live in. If we're too cold in the house, we can readily

Fish live in a totally different environment than we do. We create an artificial environment for them to live in. If we're too cold in the house, we can readily imagine that the dog is also too cold. If we have insufficient oxygen, then the dog may well have insufficient oxygen. If our air is filled with toxins, then of course the dog's air is filled with toxins. We have little or no direct concept of what the environment is like for the fish, and, without studying it, we don't inherently know what their environment ought to be. Superficially, fish live in water, so it's easy to assume that we can toss them into any body of water and it's fine. But of course that's not true. And the details vary amongst different fish. EDIT: I think also, people tend to view fish as decorations. Or more to the point, aquariums with fish are decorations. You have to maintain them similar to how you have to water houseplants. The fact that they are animals and not plants might be seen as a minor detail.