"Ta ryba jest stara."

Translation: This fish is old.

[michmo]

Ryba jest nieświeża

[vytah]

Why are you assuming the fish is dead?

[michmo]

yes.In Poland its sounds funny

[Antosh17]

Agree, the context when "ryba jest stara" is rare.

[chce_polski]

This is an old fish! C'mon!

[Jellei]

Please see here, Part 6: https://forum.duolingo.com/comment/28029344

[U_pinu]

Is "stara" in the nominative case? Shouldn't it be instrumental? (hence starą?)

[Jellei]

Instrumental is needed for a noun phrase. An adjective on its own will just use Nominative.