C´est un poisson au café. Why that´s not correct?
Your sentence says "It is a fish at the café". I am only guessing at the meaning of the sentence the way it is written, but I think the intended message is that the café serves fish ("There is fish" as opposed to "There are fish").
Hope that helps!
Sorry but I had to laugh ^^.
Basically @Archegui, your sentence means "It's a coffee-flavored fish". That sounds disgusting enough xD.
No offense, I'm not mocking you, it's just that the sentence is so far from the original meaning ^^.