Unfortunately, the fish keeping hobby is far from the cause of the decline of the oceans.
Yes, there were ( and probably still are) areas in the world that used chemicals to collect the fish we keep but there are less of those areas now. HOWEVER, environmental reasons are more to blame for the decline of the oceans than fish collectors are. It's happened here in FL and was directly caused by human activities NOT associated with tropical fish collecting. Yes, human activity is a major cause of the decline even if you don't believe in climate change being a cause but it's not from collecting fish. Look at the decimation of the shark population solely for the fins to make soup. The problem is that without the top predators, the system collapses and it's shown by the areas where there is no shark collecting and those areas are teaming with fish life.
Now, if you want to be a responsible aquarist, don't try to keep a fish that grows to 20 lbs in a tank that only holds 50 gallons of water. Don't keep a fish that needs open water to swim in in a tank that is highly decorated because it looks nicer that way. Don't keep trying to mix the fish you want instead of mixing the fish that can be mixed.
Threads like this are good for discussion but without the facts, they are not really beneficial. Here's some articles you should read:
Massive 40% decline in ocean's phytoplankton puts entire food chain under threat | Daily Mail Online
Loss of Biodiversity and Extinctions — Global Issues
(This last one is science and not opinion.)
It's not because of the tropical fish industry.