Hard to say. As a first try I would accent it differently and use some mimics: Ta ryba jest złaaa! But how to write it expressly? For other possibilities I used synonymous dictionary: http://synonim.net/synonim/zły
Just look how imprecise and universal the word "zły" in Polish is.
Ok. So now other possibilities:
Ta ryba jest diabelna, szatańska, piekielna, demoniczna. (devil-like, devil-like, infernal, demonic)
One of my favourites words in Polish: złowroga = zły (evil,bad) + wrogi (enemy). Ta ryba jest złowroga
Podła ryba - this is good (I mean, to denote sth as evil) if one talks about humans, in context of everything else it means cheap and of bad quality
Bezecny (another word I like), niegodziwy can be use only with humans
The instrumental case is not needed when it is an adjective by itself. Zła is an adjective by itself so it stays in the nominative case.
Instrumental case is used when the verb być describes another noun, and any modifying adjective always matches the case of the noun that it modifies.
Jestem złym człowiekiem.
Yes... but generally the problem is, that the adjective "zły" is surprisingly difficult in usage, despite the fact that it seems to be so basic. It has been introduced too early, and frankly, most of the sentences with it are... bad.
This does not mean that the fish is 'not fresh', for example.
I had to build the sentence thanks to the labels, but could not choose "bad", when "evil" was expected... According to me it's really weard to say about a fish that "it is evil". But neither English nor Polish are my mother language. Could someone check if this translation is really correct?
It is correct, it's just that the sentence is... bad. "zły" is a surprisingly difficult word to teach, despite the fact that it seems so basic. So frankly, at least in my opinions, all the sentences introducing it are... well, bad. Because it was just too early in the course.
Each one is used for different genders of the noun it's referring to.