* If you mean the colorful, small fish commonly kept in aquariums (Synchiropus splendidus), they are mostly immune to predators due to their toxic skin. However, some larger fish like groupers, triggerfish, and eels may try to eat them.
* If you mean the "Mandarin fish" in the genus Siniperca, found in Asia, they are a popular food fish themselves! They are eaten by humans, but also by larger fish like catfish, carp, and snakeheads.
To provide a more accurate answer, can you tell me which type of mandarin fish you're interested in?