Thank you both for your replies. I can't imagine a firefish chasing a tang! The tank is an ea reef 600s, the display tank volume is about 145 litres, total system volume 167 litres. In terms of fish I have two Picasso clowns, a silver belly wrasse, a sunburst goby (paired with a pistol shrimp) and the two fish I've posted about. The purple firefish is my final fish. My lfs said this stocking list was fine but now I'm concerned! I acclimated the firefish before adding it to the tank and adjusted my rock so that there were gaps and caves. The McCoskers occasionally chases my silver belly but it's being persistent with the firefish. Hopefully they'll settle soon.