i think its einen because 1. its talking about 'A' fish 2. the phrase 'a fish' occurs at the end of the sentence so its becomes the Accusative so that means its the einen form...but If you start the sentence with say A fish is healthy/good to eat then it would be ein fisch because the -en (einen) part only happens when its the 2nd part of the sentence not the start...Please is this correct, I am making a learned guess here??
Words do not become accusative just because they are at the end of the sentence. If you read the theory part in Basics 2, you will find this example: "Den Apfel hat das Mädchen." Here, "den Apfel" is clearly the accusative. The accusative simply means that somebody/something else is doing something to this object. Even though in this example the man's action on the fish is more passive, because he "is owning" the fish.
In this example, einen is correct.
der Fisch is a masculine noun, so the indefinite article would be ein if it was the subject of the sentence and therefore in the nominative case.
However, in this example, der Mann is the subject of the sentence. He has a fish which is the direct object of the sentence, so it is in the accusative case. In the accusative case the indefinite article takes an -en ending for masculine nouns, so it becomes einen Fisch:
Fish is a countable noun - one fish, two fish, etc. It can be an uncountable noun when referring to fish as a food, but this is also true of Fisch in German:
In this example, the indefinite article is used in German and should be translated to English to keep the same meaning:
Der Mann hat einen Fisch = The man has a fish (meaning the man has a single fish)
Leaving out the article would be OK, if it doesn't exist in the original:
Der Mann hat Fisch = The man has fish (meaning the man has some fish)
I think the English sentence "the man is having a fish" would mean that he is eating the fish right now. "The man has a fish" and "der Mann hat einen Fisch" simply means that he has one. Either he fished it and is holding it, or maybe it's swimming around in his aquarium at home. :)