Are the fish you currently have all just babies? I imagine not if the tank is 600 gallons. I'd guess any true Hillari available would be on the pretty small side. I wouldn't hesitate to add a single one to any of my tanks. I'm guessing if I added a single fish to the same tank vs a big group (started with 7) then there wouldn't have been casualties. Those were on me, my fault, should have moved the Hillari once they were similar sized to the tank mates and the tattered fins started, but have been so busy trying to get things back up after the move that nothing happened fast enough. The Hillari simply outgrew the tank mates and then 4 on 1'd them. Really don't think a single fish would be an issue in most cases.