And how would you know that the fish not using his fin well means the OP doesn't like the way the fn looks? Don't be so presumptuous.
BOTTOM LINE: The fish has a bad fin that prevents him from swimming properly, which can likely be fixed. Would you not help any of your pets improve mobility if you could? If your dog was hit by a car and his leg was broken, wouldn't you take him to the vet? The vet can set the broken bone, but it would hurt, and dogs can get around on three legs, right?
Stop making it out like the OP wants to hurt his fish for fun, he's trying to HELP his fish.