HELP! Old tang not friendly with new tang

[kien]

What kind of tangs are we talking about? Old and new, what tangs are they?

You can try placing a mirror in or near the tank so that the old tang is distracted by his reflection.

Ps. Why did you buy a sickly looking tang? He won't stand a chance against the resident tang. If you are set in keeping the sickly new tang I would take out and sump the old tang while the new tang acclimates to your tank.