Unlike birds and mammals (and to a lesser extent reptiles and amphibians), many captive fish are still sourced from the wild. This raises environmental concerns, as well as a lot of questions. What species are being collected? How are they being collected? How many is too many? What is the impact of their removal on the environment? And, as the lionfish case demonstrates, what is the risk of invasive populations being established outside of the native range?
I recently came across this mini-documentary about Hawaii's aquarium fishery. It's an interesting insight into how fish are collected for export and sale, and what the environmental and ethical implications are. To be clear, though, it is produced by commercial interests, so it's hardly an unbiased piece of material.