Mistake in French
"The grapes are red." is corrected as a singular translation.
It's not so much that fish is only singular as it isn't inflected when plural, so it is plural, it just looks the same as the singular form. It is indeclinable.
I would note that fish probably isn't the best example, as fishes does exist. It was historically a plural form, and retains a moderate amount of currency, though now it is mostly reserved for biological contexts when referring to several species of fish. Many normally invariant nouns do decline in this context.
There seems to be a trend, particularly with animals, that the singular form replaces the marked plural form. This probably has a lot to do with how simplified modern English has become and that many animals of human importance were always invariant.