How can you even decide to keep two fish that get larger than what most zoos would be able to keep, in a 40 gal tank.
Unfortunately aquarium stores don't often consider animal welfare and will sell you something that grows to 6ft right next to something that'll stop at 2 inches. Probably more of a consequence of the trade than the individuals conscious choice. Imo they shouldn't sell anything that needs 100+ gallons when full grown as almost nobody is running a tank that large. (With exceptions, but exotic tanks can afford exotic imports).