You don’t have to name your fish, but it makes the fish-keeping experience more enjoyable. When you name your fish, you notice their personalities more and recognize them as individuals.  But fish can’t hear very well between water and air. Your voice sounds muffled to them, and they won’t be able to discern words from general sounds. Even if they could, fish aren’t as advanced as other animals — like mammals — so they probably wouldn’t learn their name.