Interestingly, LDOCE (5th version) also suggests that the plural form of fish can be fishes.
However, it doesn't mean that it would be a nice idea to use fishes because it might sound strange (to some even uneducated) but not unique. If you speak in your own way, fishes might be part of your idiolect.
Moreover, we know of some nouns (like in German) that have two plural forms, and their meanings might differ.
brother → (1.) brothers, (2.) brethren
spirit (the qualities that make someone live the way they do, and make them different from other) → spirits (the way someone feels at a particular time, for example if they are happy or sad)
...though I see no lexical contrast between fish and fishes, but if you want to express fish in plural form, here's an alternative:
shoals or schools of fish