Best way to create school of fish?

I have the ability for my character to dive underwater and swim. It is super cool but looks pretty dead underwater. I need to have some marine life like fish but I don't want to slow the game by having tons of fish. Should they be prefabs and just instantiate a bunch of them? or animated billboard sprites or what?. I won't be interacting with them, they are just scenery and a fish can be extremely low poly