That is a juvenile/baby regal angelfish. Juvenile angelfish tend to have different colors & patterns compared to their adult counterparts (they seem to be a lot more "stripey" - my scientific terminology LOL). And angelfish can get HUGE from what I've seen on my diving adventures - I think they are also very aggressive to one another.
Oh that's so cool ! I want a reef tank but is it hard keeping saltwater?
Also what kind of angelfish is this do you know? Are they easy to care for?