Translation:One fish, two fish, red fish, blue fish
I tried four online dictionaries, and they all said the indefinite plural is "fisker". The complete declension: (en) fisk – (den) fisken – (flere) fisker – (alle) fiskene (https://www.dinordbok.no/norsk-nynorsk/?q=fisk )
https://no.wiktionary.org/wiki/fisk differentiates between the animal, which is countable and uses the plural forms, and fish as a dish (food), which is uncountable.
https://www.dinordbok.no/norsk-nynorsk/?q=fisk does not explain about countable/uncountable, but the examples given under 'Eksempler på bruk' → 'Vis mer' seem to show that fish is also uncountable when used in a generic sense: "There aren't any fish living in this river anymore" = "Det lever ikke lenger fisk i denne elven". But as soon as we are talking about individually discernible fish, even in a group, the plural is used: "Little fish are swimming in the aquarium" = "Små fisker svømmer i akvariet".
"Two fish" is obviously countable and the correct Norwegian form should thus be "to fisker" – at least in standard Norwegian.
Dr Seuss is also the author of The Cat in the Hat https://en.wikipedia.org/wiki/The_Cat_in_the_Hat
I'm English and I've never read any Dr Seuss either. I've heard of "The Cat in the Hat" but not this fish book.However, whenever there's a bizarre reference I'm learning to look up the discussions. where I learn as much about American popular culture as Norwegian, perhaps (?)