I would comfortably put a singular sunburst in a 25 gallon. They don't have the same roaming swim style that the Pseudanthias genera does. Single specimens of other species that stay under 4" are possibly acceptable as well. IME, anthias don't swim nearly as much when they're alone as they do when they have a group. When I lost two of my three tricolors, the remaining female took to just hanging around a large monti cap in an high-flow area and stopped cruising the tank unless it was feeding time. For what it's worth, my tank is a 125.