Freshwater aquarium - fine
Saltwater aquarium full of captive raised corals and fish - fine
Saltwater aquarium full of wild net-caught fish - fine
Saltwater aquarium full of wild net-caught fish and harvested corals - questionable
Saltwater aquarium full of wild cyanide caught fish and wild harvested corals - bad!
Fish are a renewable resource - if they are managed well (most aren't of course). If you eat fish, then it's hypocritical to question someone's aim to keep them as pets, IMO.
The average price of reef fish sold into the aquarium trade is many $$$ per pound for the fisherfolk. The average price of reef fish sold into the live food fish trade (caught w/ cyanide) is much less, and less still right to the fish market. Which would you prefer?
Cheers
James