Fish is the plural of fish.
To be more precise, "fishes" is used, but it means "kinds of fish."
In other sentences the preposition of "there" was not needed and was marked wrong if it was included in the sentence but in this sentence the preposition is there and if not written down is marked wrong. What I mean to say is that this lesson is not consistent.
what would stop this being the general 'fish' as opposed to those specific fish? would that be written differently?
No problem with the answer but I do question it. The meaning of oda is movement in a direction leading to a place. If we say, the fish in the river swim there, we aren't saying that the fish are swimming to the river because, presumably, they're already in it. In this question, what is it that the fish are swimming to? I would have thought ott was more appropriate in the sense of the fish being located there and swimming rather than swimming to a particular place.