>>46925916>>46927373>>46928076Yeah, both of these. Feebas' Dex entry makes it clear that it's a hardy fish that can be found in any kind of water, just like Magikarp. They can also "eat anything", so food supply isn't an issue. It's actually a relatively COMMON kind of fish, insofar as they reproduce consistently, aren't hunted by predators because they look unappetizing, and aren't pursued by most trainers because they're weak. More than anything, they seem to be rare because they hide at the bottom of bodies of water, among weeds. Once found, they're pretty easy to catch, because they're dumb and slow. Knowing that Feebas evolve into Milotic actually seems like somewhat obscure knowledge, in-universe? Or maybe people just know it takes a lot of work, just like Gyarados.
Pokemon kind of doesn't differentiate between salt and sweet water for its marine life. They care more about biomes, and putting water types in appropriate environments more than they do specific bodies of water-- so you'll never find a Bruxish in a swamp. As far as Feebas is concerned, in the games, it is primarily a freshwater fish, found first almost singularly in rivers and ponds, but that gradually expanded. I'd wager Brooklet Hill, which is a series of small ponds leading into a lagoon, is brackish towards the bottom, and the Roaring-Sea Caves of Galar are definitely salt. Considering saline levels and applying environmental considerations to Pokemon distribution can be enjoyable and help flesh out the world, make it seem like more of a real living ecosystem, but as a whole it seems water types are able to sustain themselves regardless of water, if only so that you don't need certain Pokemon to Surf in certain bodies of water.
My guy here is big into fishing, so I read up what little lore there was on it because I wanted to nail down the means by which a Feebas could be caught. It boils down to dumb luck, really, which means someone that fishes consistently will eventually catch one.