As many fish die due to hobbyists and the untrained, many, many, many more die in the wild. Most fish that are suitable for aquarium keeping are just feeders in the wild -- they are very low on the food chain. As topsy-turvy as it sounds, the ones that make it to the aquariums may be the lucky ones. Regular meals, no predators, and owners who medicate them when they catch a disease... you don't get any of those things in the wild. Except in cases where the native populations have been decimated -- the red tail black shark is pretty much extinct outside of aquariums today, for one example -- catching and selling wild fish isn't all that terrible. Lots of cardinals are wild caught and sold, for example, and their population levels are fine. And, again, those that are caught and end up in an aquarium are probably the lucky ones. I don't have any hard figures on it, but I'd be very surprised if more than 1 in 1000 fish in the wild live out their entire natural life. It's probably more like 1 in 10 000 or even 100 000.