Q: Do fish ever get thirsty? Why or why not?
A: This is an interesting question; I have never thought about it before and looked a few things up.
It is doubtful that fish would ever feel thirsty as such and decide to "drink more." The concentration of fluid in their bodies as opposed to the water surrounding them is under a form of automatic control, so the fish has no conscious control of it.
Most fish that live in the sea have blood that is less concentrated in terms of ions than the surrounding water they live in. Through osmosis these fish will lose lots of water into the sea. To stop them from dehydrating, bony fish "drink" or consume lots of water all the time, I guess sometimes more than at other times as a response to the ion concentrations in their bodies -- you could call that thirst.
Fish have special adaptations of their gills that enable them to excrete the excess salts they are taking in from the surrounding water and so maintain the concentration of their body fluids.
I hope this answers your question; it is more complex than you would first think!
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