"Ty ryby jsou velké."

Translation: Those fish are big.

[CassieMALI]

Not certain if you want to amend this to include the plural for fish as fishes

The term “fishes” on the other hand, is used to refer or describe the plural of different species of fish (more than one species of fish). So, fish can be used in a singular form when describing one fish, as well as, in a plural form only when describing two or more fish of the same species.

[Fabiola-609]

I will report "fishes" right now. In the big acquarium of the Sea Park, there are lots of fishes of all kind, measure and colour: FISHES is an English existing word. We eat fish at the restaurant/There are other fishin the sea/He caught three fishes. Sometimes "fish" as plural is better, but not always.

[goldregenb]

I would forget about the word "fishes" altogether. It is not used. Neither colloquial nor any other way. When I go out fishing, I hope to catch a lot of fish, of all kinds. When I go to a restaurant for seafood, I look forward to them having many different fish dishes to offer. And an aquarium has thousands of different fish from all over the world on show.

[Gilbert334611]

Thank you, as a Germam I didn't know this.

[itsmeyash31]

Is ryby in accusative case here?

[VladaFu]

Nominative, it is the subject.