Are mandarin fish freshwater fish

It depends on which you are referring to, there is both a freshwater and a marine fish that carries the name mandarin, the two species have no relation to one another what so ever.

There is a Freshwater Fish called a mandarin fish or a Chinese perch that can be found in china.

There is also a mandarin goby that is a Saltwater Fish that dwells in reefs, and lagoons feeding on small pods and crustaceans.