"One fish, two fish, red fish, blue fish"
Translation:Én fisk, to fisk, rød fisk, blå fisk
It's a rhyme, and the name of a Dr. Seuss book (see the image up top).
The adjectives are in their correct form, indefinite m/f, to match up with the noun "fisk". The neuter forms of the adjectives would be "rødt" and "blått" respectively.
"Fisk" works much like "fish" in English, in that the singular form is often used in a plural sense. "Fisker" would only be a strongly preferred plural in cases where each individual fish was in focus. When using it strictly as a mass noun, "fisk" would be preferred, and in all other contexts you have two options.
For example, if someone were to ask you how much fish you caught on your trip, both "Jeg fikk fem fisk" and "Jeg fikk fem fisker" would be correct.
Since fisk might be the plural sense, could we also say "røde fisk, blåe fisk" here? (from the book cover illustration, these phrases appear to be labeling a single red fish and a single blue fish, but I mean, in the absence of an illustration, might we use the plural forms...?)
Interesting... in English, the rule I usually see is that one says "fishes" when referring to different species of fish. I don't think the distinction that you make, in which "each individual fish is in focus", would have occurred to me as a use of the English word "fishes", but it does sort of make sense in that context.
That said, as you point out, using "fishes" as a plural in English does not sound stilted or archaic or weird. It's just less common than using "fish" as a plural.
Back to my previous point, though: is the use of "fisker" also preferred when talking about different species of fish?