I saw several in a large tank at a New Jersey aquarium and have seen several in the same tank at fish stores. They say (live aquaria for example) to introduce royal dottybacks at the same time to prevent aggression. If you do keep more than one gramma, that might be the approach to use. One site (reeftime.com) said to use a 6' tank if you want more than one royal gramma, and they aren't a mated pair. I have both an orchid dottyback and a royal gramma in a deep 3' tank and hoped they would eventually get along. The orchid dottyback chased the Royal Gramma a little at first, but my tank is loaded with live rock hiding places. I placed a mirror next to the tank away from where the Gramma hid to keep the Dottyback busy; it worked. My royal gramma now tends to pick a location near a hideout and not move more than a foot from it. When other fish swim buy, it opens its mouth and faces them, but never chases them. It has a big mouth for a small fish. I saw on reefs.org once that the dominant fish will change sexes to male, like in many other fishes.