I think most of people will say that it is not that cryel if the fish is lucky to have a good owner as it can have food freely and a cozy home(cozy home for the people who see but not for the animal who will put on it)... In the wilderness that pet will need to strive hard. And in people perspection they think that it more convenient to animal to keep them cause the animal/pet will no need to risk their life for food or they will be more safe from it's prey...