Red fish are called "fish" because they are, well, fish! The word "fish" is a general term for aquatic vertebrates that breathe through gills and have fins.
There are many different types of fish, including some that are red. The term "red fish" itself can refer to a variety of fish species, such as:
* Red drum: Found in the Gulf of Mexico and Atlantic coast of the U.S.
* Red snapper: Popular food fish found in the Gulf of Mexico.
* Red grouper: Another popular food fish found in the Atlantic Ocean.
So, the reason red fish are called "fish" is simply because they belong to the category of animals we classify as fish.