You are correct but both fish and fishes are accepted as correct in English. If you want to answer to be just fish, the phrase should be more specific. I have over 20 years of schooling (high school, undergrad, grad) and have taken several English writing classes. Fishes has always been acceptable as a plural form of fish.
After a quick google search I have found a picture of a fish drinking orange juice: https://9gag.com/gag/ajrNWBq/fish-drinking-orange-juice-from-a-baby-bottle3
why is it keinen and not nicht?
If the verb has a direct object which is indefinite, you will generally use a form of kein to negate the sentence.
if it says " i dont drink orange juice" it will be ich trinke orangensaft nicht wont it?
No, it won't. It would be Ich trinke keinen Orangensaft. (with kein and with capital O on Orangensaft.)
Ich trinke Orangensaft nicht would negate the verb: "What I do to orange juice is not 'drink', ..." -- and the sentence would feel incomplete if you don't say what the actual verb should be.