Ryba jest nieświeża
Why are you assuming the fish is dead?
yes.In Poland its sounds funny
Agree, the context when "ryba jest stara" is rare.
This is an old fish! C'mon!
Please see here, Part 6: https://forum.duolingo.com/comment/28029344
Is "stara" in the nominative case? Shouldn't it be instrumental? (hence starą?)
Instrumental is needed for a noun phrase. An adjective on its own will just use Nominative.