Build an isolation box out of eggcrate and zap straps. Place box inside the tank and add the new tang. The old fish will spend the next few days getting all excited at the sight, smell, etc. of the new fish but not be able to touch it. When everyone is calmed down, you can release the new tang. I've used this many times in the past and plan to do so again next week.
Also, add a mirror against the tank and try turning off the lights for a bit. These tend to distract or calm down the agitated original inhabitants. I also tend to feed extra.
Anthony
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