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).
I have three friends with 110+ gallon tanks…
To be fair though, none of them have large breed fish, nor do any have any rare or difficult fish. They run larger tanks because it’s far easier to keep them stable. Takes longer for temp, ph, nitrate level, etc. to change.