Aquatic creatures: Saltwater vs. Freshwater list, what if one enters the other?

Also, I am trying not to go to sleep because then I would have to wake up on time and do things tomorrow, and making a list of fresh/salt creatures sounds like the perfect waste of time to me.

Freshwater only: Bullywug (I'm assuming an amphibian's permeable skin is just as sensitive to salt as a freshwater fish's), Crab, Frog, Giant Crab, Giant Frog, Giant Toad, Quipper, Swarm of Quippers

Saltwater only: Crab, Giant Crab, Giant Octopus, Giant Sea Horse, Giant Shark, Hunter Shark, Octopus, Quipper, Reef Shark, Sea Horse, Swarm of Quippers

Transitional: (These are the ones that can live in either, but take a while to adjust) Kuo-Toa, Merfolk, Merrow, Sahuagin, Sea Hag (yes I know it says SEA hag, but a lot of adventures have them in lakes and such)

Any other creature not listed can survive just fine in either water. (E.g. Aboleths -- they are covered in mucous; Chuul -- they are covered in chitin; Water Elemental -- they are water; Killer Whale -- they are covered in blubber; also why are we calling them "Killer Whale" again when I thought the proper term was "Orca?")

Note that a few creatures appear on both the freshwater and saltwater lists (e.g. Crab, Quipper) because these stat blocks represent multiple species but most of them either stick to freshwater or saltwater and don't transition between them. Quippers are generally thought of as analogs of piranhas, which are freshwater, but I think in a D&D world there will be plenty of saltwater quippers.

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