Hi. I just recently ran into this same issue. I have a 70 gallon cube and had my kole tang for 2 months with 2 purple fire fish, a exquisite wrasse and 4 chromis. All the fish got along no problems. I introduced a diamond goby and from the start the tang pestered the goby non stop eventually killing it. I then bought a second diamond goby and the same issue persisted eventually killing it as well. I eventually took the tang back to lfs because it started nipping at the fire fish and wrasse as well. As much as I liked the tang I just couldn't keep it with its aggressiveness. I hope this helps.