Different animals adapt to the climate of their environment (eg thick fur, different coats for summer/winter, etc). But for the most part, fish look fundamentally the same (scales, fin placement, etc) across cold and warm water species whether salt, brackish or fresh water.
In: Biology
The adaptations you see in mammals (thick fur, etc) happen precisely because these animals mostly _don’t_ have adaptations to live at different temperatures. Mammals have to keep their core body temperature within a pretty narrow range, so if they live in cold climates they need lots of insulation to make this possible, and if they live in warm climates they don’t need heavy insulation.
Fish, on the other hand, aren’t warmblooded (with a few exceptions). Instead of generating heat on the inside and using insulation to try to stay a different temperature from their environment, they are just directly adapted to live at various different temperatures. Some live better at warm temps, others at cold temps.
Fish have adaptations for different water temperatures, but they aren’t the sort of adaptations you can see. Instead, it’s things like having altered proteins, or different cell membrane chemistry, etc. Biochemical differences that make sure the stuff in the cell works well at…well…at whatever temperature the fish lives at. And since their cells are adapted to the right temperature, they don’t need any visible insulation to help them keep their insides warmer.
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