In the English language the noun "fish" is singular and plural. The only time you would correctly use "fishes" is in the verb form as in He fishes often. Fishes as a noun is never correct although people use it that way. There are a lot of words we use in English that sound comfortable but are extremely incorrect.
The plural of fish is either fish or fishes.
Fishes is always plural and is used when describing more than one species of fish.
Examples: There are a lot fishes in the sea. (plural) Ichthyology is the study of fishes. (plural)
Fish is the plural and the singular, and if you were only talking about one type of fish they would all be fish. Fishes is a plural plural, I.E. more then one group of more then one. In the sentence "All the fishes in the seas", you are refering to more then one type of fish, and more then one fish in each type. If you had only 1 fish of each type, then would be the only time that fish and fishes are interchangeable. the same applies to words such as sheep and beer.