I don't believe that there is anything that you are looking for exists for such a small nano tank environment. A majority of the fish in the saltwater side of things need large amounts of water...be it for swimming, dilution of pollutants in the water column from their very specialized feedings...whatever the case is.
At the end of the day, a 8 gallon tank isn't enough for a vast majority of fish to live singly. These tanks in the nano community, that are long term successful, keep either a small goby that stays in the sand/rock or is solely an invert setup with a single invert, mated pair sort of deal.