No, you cannot shoot a fish out of a gun safely or realistically. Fish are living organisms, and their bodies are not designed to withstand extreme pressure or force like a bullet. Even if you try to load a dead fish into a gun, it would be destroyed instantly by the force and would not fly like a bullet. Also, guns are designed for solid, compact ammunition. A fish’s soft, irregular shape would jam or damage the barrel. Scientifically, it's not practical or possible. It’s probably just based off a joke or meme, and not real physics.