It is always a good idea to check out the comments in the thread before posting, because this has already been discussed. It can't be singular, because it says "the fish swim". If it were singular, it would say "the fish swims".
Another unclear thing . The fish is a collective term in English and yet you often say in english fishes(which nobody says) so I put it in the singular now and it seems you wanted it in the plural. This is confusing.