It is rare for an uncountable noun to have a plural form, but fish and fishes is one of the exceptions to the general rule.
While fish is the most common form of the noun, fishes can be correct in specific contexts.
Broadly speaking, the uncountable form refers to fish as food or a catch.
But fishes is often correct when talking from a scientific perspective about a collection or variety of fish species.
The uncountable noun
Most uncountable nouns have no plural form.
Nouns like advice, information, water, and furniture never take an S.
The only way to make these nouns countable is to use a partitive or number.
He gave me three pieces of advice.
The tourist office gave me lots of information.
I always drink six glasses of water every day.
It is the same for the noun fish. It usually needs a partitive when we refer to it as food or even by the name of the fish.
There are six fillets of fish in the refrigerator.
Look, I caught four big trout for dinner!
In most cases, fish in the uncountable form is correct.
The scientific use of fishes
The plural form is a rarity in general use, but it is used in research about fish.
From a grammatical perspective, it usually indicates a number of species of fish, often in a prescribed body of water.
You most often see it in scientific journals or studies.
Of the twenty fishes in the lake, only one was considered endangered.
Study hints at how fishes in the twilight zone evolved.
There are quite a few references in the Bible to Jesus feeding the multitude.
But the use of fish or fishes in the quotes depends on which version of the Bible you choose.
And they say unto him, We have here but five loaves, and two fishes. (King James Version)
Then he ordered the crowds to sit down on the grass. Taking the five loaves and the two fish, he looked up to heaven and blessed and broke the loaves and gave them to the disciples, and the disciples gave them to the crowds. (New Revised Standard Version Updated Edition)
As you can see, both are possible to use.
In literature and prose
Many quotes use the word fishes, often to create a rhyme.
“If wishes were fishes, we’d all cast nets.” – Frank Herbert
“If Wishes Were Fishes We’d All Swim In Riches.” – Scottish nursery rhyme
“Fishes live in the sea, as men do a-land; the great ones eat up the little ones.” – William Shakespeare
Fixed expressions with fish
There are far too many expressions with fish to list them here.
But in general, these fixed expressions all use fish and not fishes.
A big fish in a small pond
Drink like a fish
A fish out of water
There are plenty more fish in the sea
Other uncountable nouns with a plural form
When you use some uncountable, or mass nouns, it is sometimes possible to create a plural.
In this case, it often refers to varieties, similar to how species are used for fish.
Tea is usually uncountable, but in the following example, it can use the plural form.
My favorite teas are Darjeeling, English Breakfast, and Earl Grey.
Beer is another that can use an S.
Our local pub serves ten different beers on tap.
One last possibility is when you delete the partitive.
Sam drinks six cups of coffee most days.
Sam drinks six coffees most days.
As you can see, when you use mass nouns, there is occasionally a way to make them plural.
Yes, fishes is a grammatically correct plural.
But only when you are talking about different species of fish, quoting the Bible, or using a rhyme.
However, 99% of the time, fish as an uncountable noun is the right choice.
I’ll end with one of my favorite quotes.
So long and thanks for all the fish. – Douglas Adams
Related reading: Backward Or Backwards And Other Direction Adverbs With An S