What's the difference?
If you are envious of someone or something, this means that you want something that they have, but this does not mean you feel angry or bitter about it or that you want to be that thing or in its situation. You just wish you could be or have been in that situation: He came back from Australia and told us all how wonderful it was. I was quite envious.
Now if you are envious of someone to the point that you feel angry, bitter and resentful towards that person, then you are definitely jealous: I was jealous that she could choose any boy she wanted. His arrogance and his friendship with such a famous person offended colleagues who were perhaps jealous of his natural talents as a performer.
The easy answer is that the plural of fish is fish (if you are talking about the meat, then it is only fish), but fishes can be used to talk about many species of fish. So if you see three fishes, you saw three different kinds of fish; if you say three fish, you saw three individual fish (which could be the same or many different kinds, but this is not important).
So here is an example in conversation: Greg: I saw three big fish swim towards me! Sid: What fishes were they? Greg: I don’t know. I think one of the fishes may have been a carp.
But if we want to say that something belongs to the one fish – its mouth, for example – you would write: The fishes’ mouth opened and closed.