Actually aquarium keepers are a large threat to reefs. Look up dynamite fishing, or cyanide fishing for some examples. Fish are often caught by people below the poverty line through ways that we would would not agree with if we saw them, then sold to companies where the majority of them die. Then the healthy ones end up in our fish tanks.
While I do not think aquarium keepers are the biggest threat to coral reefs. In most cases we are certainly hurting them.
However, a certain number of species a strong case can be made for keeping them in captivity, however I have noticed that many people interested in marine fish often are not as interested in breeding them as freshwater hobbyists.
Not that freshwater hobbyists are entirely blameless, often fish are caught through equally harmful methods (such as poisoning (especially with fish such as oto cats)) but often freshwater hobbyists are usually more interested in breeding thier fish in home aquariums (notably fish like Ameca spledens and some others only exist in captivity now).
Anyway my view if its wild caught at least make and effort to try and pursue breeding it in your aquarium so you can prevent harm to aquatic environments (fresh or saltwater).
Cyanide is one of the cases I stated as well. My point was not to say reef euthusiasts are not compelely guilt free on the topic, just that its up to all of us to educate ourselves and steer clear of places that sell living creatures caught by those methods. I think given the choice, most hobbyists prefer tank-raised and aquacultered as the specimens are hardier and more accustomed to the aquarium environment.
Yes a lot more FW hobbyist like to breed, particularly live-bearers. However I know quite a few scenarios in which it results in an over stocked tank, deformities brought on my inbreeding, and can still result in poor overall health for the fish.
I think there are detrimental effects on the environment from both forms of aquarium keeping. Some more prominent than others. There was recently an article in the National Geographic citing areas where freshwater fish natural to their locations have been nearly wiped out, either intentionally to stock the waters with more fisherman-friendly fish or by the introduction of non-native species. Oscars are a great FW example of this. Oscars originate in south america, in small rivers that connect to the Amazon. Because of irresponible FW owners, they now thrive in certain areas of Florida.
So if anything can be said, it is this. Do research, be responsible as possible.
When it comes to domesticating wild animals or recreating their habitats, it always comes at a price. The price paid by the environment can be limited through education, but not eradicated.
This goes for both FW and SW owners, so don't attempt to cruxify one without first taking a good glance at your own form of the fishkeeping hobby.