Do fish in an ocean sleep at night? Do they have a circadian rhythm? How are they even able to survive in such high-salinity environments to begin with?

Fish not only sleep, some make sleeping bags (parrot fish). Most fish are hardwired for either fresh or seawater, some including Bull sharks, can freely travel between them. Aquarium fish can be observed sleeping too. For really weird biology, search for "ice fishes". Good luck 👍