The difference is that Guppies & Angels are bred by private in captivity, the sharks are not,
the sharks are endangered , whereas guppies and angels aren't
How do you believe that started? Their are lots of animals in aquaria which are near extinct in the wild. I believe ultimately a properly run sea life exhibit is an excellent thing as it promotes education and I genuinely believe that the marine biologist and zoo keepers look after the animals to the best of their ability so that they can learn not just to make profits.