The fish do not appear in an other Fallout game, except for the cut fish that were meant to be an consumable item in Fallout 2. (See here.) Fish were mentioned by an unnamed fisherman in random coastal encounter in Fallout 1 and by San Francisco citizens in Fallout 2 as food.
Fish featured in Fallout: New Vegas appeared to be virtually unaffected by radiation, possibly due to low background level of radiation in Mojave area. However, according to various wastelanders' account in Fallout 1 & 2, many fish caught in Pacific Ocean were mutated by radiation, showing abnormalities such as having three eyes or gigantic in physical size. Appearantly, Fish cannot survive in heavily radiated Potomac river in Capital Wasteland.
When the Courier asks Cass why she took up caravaning, she responds by saying she "took to it like a fish in water." The Courier has the option to say "What the hell is a fish?" She replies that a fish is like a slimy lakelurk thing that doesn't have claws and doesn't attack people. She also describes it as a bird that stays underwater, and that some of them have legs. Other dialogue options lead to Cass saying that she has never seen a real fish, only a plastic one on a wall in a bar (that used to sing, according to the bar owner and much to Cass' disbelief).