The plural of fish is usually fish, but fishes has a few uses. In biology, for instance, fishes is used to refer to multiple species of fish. For example, if you say you saw four fish when scuba diving, that means you saw four individual fish, but if you say you saw four fishes, we might infer that you saw an undetermined number of fish of four different species.

http://grammarist.com/usage/fish-fishes/

You are not alone with this. :D

Fishes is accepted as well https://dictionary.cambridge.org/dictionary/english/fish

It's masculine singular and, in this sentence, in the accusative case (as it's the direct object of the verb trinken).

Thus the masculine accusative singular form keinen is used in front of it.

Den Saft!!!

yeah, i'm going to be saying some weird stuff "Hello, my name is Quinn and fish don't drink orange juice, nice to meet you" ... LOL!

Yeh why would a fish drink orange juice anyway and cows wear hats? Makes it very interesting

Fische trinken keinen Orangensaft! -> Fish drink no orange juice Fische trinken nicht Orangensaft! -> Fish do not drink orange juice

Yes, you're right.

And your sentence works as a translation. It's less common, however, depending on the context. You'd normally use this phrasing to emphasise the verb, e.g. "Wir essen die Pferde nicht, wir reiten sie" - "We don't eat the horses, we ride them"; as opposed to "Wir essen keine Pferde!" = "We don't eat horses!" = "Horses are not something we eat".

Because Orangensaft is masculine (like der Saft).

keine would be appropriate for a feminine noun or a for a plural one.

As the word Saft is masculine (der).

Or perhaps 'Fische nicht trinken Orangensaft' - but Duo is does not accept either of these.

Since the English "fish" is a plural here, you need the plural in German, too: singular "der/ein Fisch", plural "(die) Fische".

(In case you're not familiar with this: in some contexts, the English plural of "fish" is "fish", in others it's "fishes". In German, we just use "Fische" in every context, except when it's food, e.g. as a headline in a restaurant's menu: "starters, pasta, fish" = "Vorspeisen, Pasta, Fisch"; "I don't eat fish / pork" = "Ich esse keinen Fisch / kein Schweinefleisch".)

So you need third person plural for the verb as well: "they drink" = "sie trinken" - which gets you: "Fische trinken keinen Orangensaft."

"trinke" is first person singular: "I drink" = "ich trinke".

If you you use Fisch it would be trinkt not trinke. That is probably why it was wrong. Plural would be Fische trinken.

If you want to use nicht, it would have to be Fische trinken Orangensaft nicht. (And because you're negating the verb, it would imply that they do something else to the orange juice -- spill it? smoke it? sell it?)

Since the verb has to be the second thing in the sentence, Fische nicht trinken is impossible since you would have both Fische and nicht in front of it, pushing the verb to the third position.

As you wrote, the gender of a compound word is determined by the gender of the last component - so the gender of Orangensaft is the same as the gender of Saft: masculine.

Orangensaft is the direct object of the verb trinken, so it has to be in the accusative case in this sentence.

Thus you need masculine accusative here: keinen.

kein would be masculine nominative (or neuter nominative/accusative).

And finally, saft and orangensaft are not words in German -- you have to capitalise them.

Because Saft is masculine (singular), and therefore so is Orangensaft.

keine would be for a feminine or a plural noun, but here you need masculine accusative singular keinen.

Fische trinken Orangensaft nicht! would usually mean that you're negating the verb -- what they do to the orange juice is not DRINK it, but instead they do something else to it.

In general, translate nie, niemals as "never", nicht as "not".

Also, Orangensaft has to be capitalised -- it's a noun.

If the negated sentence has an indefinite (i.e. with indefinite article or no article at all) accusative object, negation is done using a form of "kein", not "nicht".
So the negation of
"Ich will den Orangensaft" is "Ich will den Orangensaft nicht." but
"Ich will Orangensaft" becomes "Ich will keinen Orangensaft".

There are two exceptions of this rule:
1.) If you move one part of the sentence to the beginning, in order to emphasize it, "nicht" is used again:
"Orangensaft will ich nicht."
2.) If you want to particularly negate a specific part of the sentence, in order to contrast it with a different alternative, then "nicht" is used in a position directly before this part of the sentence. But in this case the sentence must have a continuation. E.g.
"Ich will nicht Orangensaft, sondern Apfelsaft."

I think the nicht would have to next to the verb, but they prefer keinen. Let's say the fish did something else with the orange juice. Then maybe you'd use nicht. I was hoping the reason was they preferred Apfelsinensaft

We don't say die Orangensaft.

Saft is masculine (der Saft) and therefore so is Orangensaft: der Orangensaft.

not really.

It sounds odd to me. Use kein- before indefinite nouns.

The rule about "nicht" in the end reversing the sentence doesn't exist.
The problem with your sentence is not the position of the verb, but the usage of "nicht". If the sentence to be negated has an indefinite accusative object (i.e. one with either an indefinite article or no article at all) you don't use "nicht", but a form of "kein", which always stands directly before the word.
So it is "Fische trinken keinen Orangensaft".

Because "Fische" is indefinite. Sentences with an indefinite (i.e. with an indefinite article or no article at all) are usually negated using a form of "kein", not "nicht".

Because die Fische is "the fish" -- it refers to a specific group of fish.

But the English sentence has just "fish" without an article, referring to fish in general.

It sounds a bit odd to me, but I've added a version with "any" now.

"kein" takes the same endings as the indefinite article "ein" and all the possessive adjectives. So "keinen" is accusative masculinum singular. (it could be dative plural as ell, but that doesn't fit here).

Because Orangensaft is masculine, not feminine or plural.

keine would be correct for feminine accusative or plural accusative, but not for masculine accusative.

Normally this is not the usual way in German. It is only possible to use "nicht" if the sentence has some kind of continuation that presents an alternative. E.g. "Fische trinken nicht Orangensagt, sondern Bier" ("fish don't drink orange juice, but beer"). Without such a continuation it is grammatically wong not to use "kein".

That link tells a strange, but true story. בס״ד

Ist jetzt hinzugefügt. In einem solchen Fall besser melden.

If you replace Orangensaft with "Wasser", it's "kein", but with Orangensaft it's "keinen".

That's right.

Wasser is neuter, so you need neuter accusative kein Wasser.

Saft (and therefore Orangensaft) is masculine, so you need masculine accusative keinen Orangensaft.

For a feminine word such as Limonade, you would have feminine accusative keine Limonade.

It could be that I am referring only to only one fish

No, it could not.

“Fish don’t” can only refer to multiple fish.

If it had been one fish, it would have been “a fish doesn’t” with a different verb and with an article before the word,