One other thing I will add here... You could probably add 6 more small fish. I don't have any science to back this up, but I have observed in my own 55g tank that a heavy fish load did wonders for overall tank stability.
That said, I also had a LOT more rock in my tank than you have. Probably 80 pounds of rock in a 55g tank.
Lots of hidey holes for fish. Lots of rock to block strong flow areas. Lots of rock for the bacteria to grow on. I believe that this, plus the very heavy bio-load, is the reason I haven't really ever had any nuisance algae issues with my tank other than a short lived bryopsis bloom at about month 6 or 7.