A group of fish of the same species are called 'fish'. Two or more species of fish are called 'fishes'.
For example, a number of Eastern Australian Salmon swimming together can be called a school of fish. But if one Tarwhine starts swimming with the Eastern Australian Salmon they are called fishes.
Books such as the Sea Fishes of Southern Australia and Coastal Fishes of South-Eastern Australia use the term 'fishes', because more than one species of fish are included in the book.
If you don't believe me, believe Shakespeare. In Pericles Act 1 scene 1 the Third Fisherman says "Master, I marvel how the fishes live in the sea".
For more information, view the Wikipedia comment on Fish or Fishes?