Hard to make the argument that any aspect of the hobby doesn't have profound effects on ecosystems across the world, but whether the weight of those effects tip towards the negative or positive is somewhat harder of a question to answer. People who keep bringing up the wild catching of fish as the cardinal sin of aquarists across the world are missing some major (like slap in the face obvious) fundamental points here. Responsible collection of aquarium fish, be they marine or fresh water in nature serve to allow the local and indigenous communities to gain profit from a healthy ecosystem. Most importantly, this gives local populations the ability to make a good livelihood without having to resort to more environmentally destructive measures as slash and burn farming, illegal mining/logging/poaching etc... In the long run, allowing people to make their livings in ways that run concurrently with the goals of the conservation effort will be a much more effective strategy than asking people to put aside the realities of their daily life for the concerns of outsiders from the industrialized world who assign a higher value to the trees and animals on a given piece of land than its human inhabitants. If anyone cares to dispute these claims, please look into the work that organizations such as the MAC or Project Piaba are doing, as well as reading up on the farming of birdwing butterflies in Indonesia before doing so. As many posters have already brought up, habitat destruction is a much more serious threat to literally any wild caught species than collection could ever amount to be. Beyond that, the aquarium hobby serves as a repository for many species that are believed to be extinct in the wild, and also provides the commercial impetus for much research and development in the captive breeding/rearing of species, information that translates directly towards our ability to re-seed and repopulate ecosystems when/if it comes to that. At the end of the day, everyone is welcome to their opinion, regardless of how idiotic or poorly informed it may be, but if one would like to make bold, broad, sweeping generalizations, they would probably do well to arm their arguments with something a bit less flimsy than wikipedia.