Dear 100 Hour Board,
If I have a saltwater aquarium, and somehow the aquarium shatters and the fish fall on the floor, can I put them in fresh water for a short time to save their lives? Or is poor Nemo toast?
By "a short time" I hope you don't mean 100 hours, for your fish are certainly dead.
OR ARE THEY?!?
Most fish species tend to live in either saline or freshwater environments. Anecdotal testing by sadistic curious or allegedly informed Quora responders indicate a saltwater fish will die 7-15 minutes after being placed in fresh water. Please, for the sake of the fishes and the childrens, do not try this at home.
If you did actually click on that link, you'll note there are a couple of fish mentioned that are the haline multi-taskers of the water world, fish capable of living in multiple salinities, savvy? To put it in Darla terms, it's probably, like, the fish equivalent of not dying when you eat breakfast for dinner, which—as you'll remember—is not something just any human being can pull off without consequences (RIP Harrison).
Pancaketastrophes aside, fish capable of living in various salinities of water are know as euryhaline fish. The ways they pull of this shenanigan-ry differ, as do to their reasons. Some live in estuaries, where rivers and oceans intersect. Others may live in brackish mangrove swamps. Some, like salmon and eels, complete part of their life cycle in fresh water and part of it in the ocean. Salmon breed in mountain rivers and streams. Eels, at least European Eels, are suspected to spawn in the calm waters of the Sargasso Sea (apparently these eels aren't doing so hot population-wise, but they don't tell you that on the telly, now do they?) and return to freshwater rivers to live.
If you'd like to learn more about the fish and creatures that see fit to inhabit salinities diverse, I'd recommend you peruse further Wikipedia's entry on euryhaline fish. White perch, mollies, puffer fish, shad, striped bass, sturgeon, herring, and the superbly named mummichog are among this robust group; as are bull sharks, which probably deserve a Board question all of their own.
So are your fish dead? Depending on what you kept in your crystalline box of water: yes, no, and maybe.
P.S. Did you know? There is a whole genus of sharks that live in rivers. These aren't bull sharks, which are a sort of requiem shark (you know the type: loves the ocean, has live births, sings in Mass), this is the elusive, rare and mysterious Glyphis genus.There's not a lot known about them, and some of the information I found about them is conflicting. Wikipedia asserts there are three species, two of which can live in the ocean, one—the Ganges Shark—which lives exclusively in the Ganges and Brahmaputra rivers of India. A website describing ray and shark biology posits the genus may have as many as six species, less than half of which have been fully described.
I don’t know how long the fish would survive in freshwater, but mixing up some salt water would be pretty quick and easy to do. Just mix some water and some aquarium salt in a bucket. Just make sure you get the right ratio, the container of aquarium salt should tell you how much you need.