Translation:Ellos no quieren que comas pescado.
There is a subtle difference between "Ellos no quieren que comas pescado" (They do not want you to eat fish) and "Ellos quieren que no comas pescado" (They want you not to eat fish), in English at least. I'd imagine the same distinction exists in Spanish. For example, you could not want your best friend to move across the country for a great new job, but that isn't the same as wanting him not to go. It's a matter of emphasis. In the latter has a more restrictive sense.
I was marked incorrect because i included the article before pescado. Anybody know the rule on why to exclude the article "el" here?
I could be mistaken, but it seems that foods don't get an article in negative sentences. "Ella no come carne," for example.
It would be correct if it were a very specific sitaution or food, for example: Ellos no quieren que coma el pescado que cocinó mi tía.