Your question is difficult to answer, because I'm not at all sure that "having instead" is correct in this sentence. The appositive, "having instead been imported" should refer back to the subject of the first clause, but the subject of the first clause is, technically speaking, "none [of the fish]." When we parse it out, we get: None of the fish is native... None of the fish have instead been imported...
Our brains will tend to resolve the problem and figure out what the sentence means (The fish are imported, not native) but it still sounds very awkward to me.
Another problem with this sentence: None of the fish is/are... We do tend to make "none" plural with countable nouns. Fish (as animals, not food) are countable. Typical usage would be "None of the fish are native."
Are you sure this sentence was marked as having no errors? If so, was this evaluation made by a native speaker of English?