Aquarium

I don't know about whether it's the best place to see exotic fish , but I enjoyed the aquarium at Hakeijima Sea Paradise in Kanagawa Pref. http://www.seaparadise.co.jp/english/

There's also Kamogawa Sea World in Chiba Pref. http://www.kamogawa-seaworld.jp/english/

And Tokyo Sea Life Park also in Chiba http://www.tokyo-zoo.net/english/kasai/index.html