I put "a fresh fish" but it wasn't accepted. If one of the translations for nowe is fresh then i think it could be a valid translation.
Being a native, I find this sentence quite strange. The only case when I would use "nowa ryba" would be after buying a fish to my tank. Otherwise - świeża ryba.
Just had the same problem as ChrisElse. Is "nowa" actually ever used to mean 'fresh' or is that more a synonym and perhaps they need to look at updating the context of the word?