"This fish is white!"
Translation:Ta ryba jest biała!
No. You cannot use the "to" construction if it is followed just by an adjective.
You may think of this "to" a bit as if it was an "=" sign. If you say "This fish = a white animal", it's okay. But you can't say "This fish = white".
More info here, especially part three: https://www.duolingo.com/comment/16373167