Which is correct 'fish' or 'fishes'?
On this page...
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.

© Sascha Schulz
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?