introducing a Tang to a tank with other tanks it is always risky, specially if it is smaller than the dominant ones. Unfortunately, in my experience, adding more cover won't make any difference, as Tangs are very active and won't take cover and at the point they start hiding is already too late.
My course of actions would be these:
- Make sure you have a good fish trap, in case things go south. If the fish looks like it is giving up, it is time to remove it from the display.
- Overfeed with dry algae and avoid meaty foods, like brine and mysis. Plenty of algae will distract the other tangs and help them recover from the fights. Meaty food, in my experience, can make tangs more aggressive.
- Keep the new tang it in a breeding box, as you are currently doing, observing the health of the caged fish and the behaviour of the other tangs. The goal is to keep the fish visible to others, isolated from aggression and long enough that the older fish get used to it, but not long enough that the isolated fish will die from the stress of being caged.
- Release the tank in the display when older fish stop caring too much for it.
- Be prepared to capture the fish again, as aggression is not guaranteed to stop with this method.
- Make the judgement if it is worth repeating the process or sell the tang.
Another option, is to cage or sump the aggressor(s). One week out of the main display will reset their dominance, at lest for a couple of weeks.