Had a black goldfish that got crunched in a rockfall. It was swimming around bent at a 45 degree angle, which was pretty pitiful. Being young, stupid, and a little inclined to give-it-a-try, I netted it out, lay it on the counter, and gave it a finger-poke above and below the bend, just to see if it could straighten and was just having a muscle cramp. There was a sickening crunch. The back straightened. The fish lived and swam for years.
In the same period, I had an angelfish (freshwater) lose absolutely all his fins, top, bottom, sides, and tail, in a fight with a predator. But he kept fighting, moving his little stubs. I kept him barebottom in a small 2 gallon, put in a bubbler that would keep him from settling but wouldn't batter him, target-fed him, and the fins began to grow back. He grew it all back, the side fins first, and within a few days he had a limited mobility---had a little clubbing in the tail and a decided ripple in the other fins when they grew back, but he made it.