Very rare, to the point that few believe they truly exist. Mermaids have been featured in stories, movies (Splash) as well as animation (The Little Mermaid). National Geographic attempted a weeklong series that earned serious backlash from the audiences due to heightened expectations. While the veracity of this image hasn't been proven yet, we hope that it isn't a hoax.
Expected dietary needs would be smaller fish, likely silversides rather than the prettier fish we keep. Mermaids are said be to reef safe, and are always kind.
Image by unknown photographer