Hmmm,
Good question. How about - Start with "legal" - always meet the minimum standards in that regard, and then try to improve from that point?
I would say that it is irresponsible to buy illegal wildlife (as per CITES and the Lacy Act). Then, be sure that you meet all local laws (e.g. no piranha in New York, Snakeheads in the U.S., pelagic sharks in Cali, etc.)
Next, you develop your own solid ethics system, and operate your hobby to meet those requirements. This includes maximizing your husbandry knowledge so that you don't need to keep replacing animals, educating others about the hobby, etc. You could also support in-situ conservation efforts.
Jay