HELP! My Yellow Tang trying to kill my new Hippo Tang! What do I do?

I just bought my husband a Hippo Tang for his b-day and the people at the fish store told me it would be fine with my Yellow Tang I have had for 6 months. I added the Hippo Tang 3 hours ago and 30 minutes ago I caught my Yellow Tang hitting him with it's spikes near its tail. I freaked out and was able to grab my net and finally catch the Yellow tang in it & now I have it quarantined in there because I don't have any other place to put my Yellow. What can I do? I don't want my Yellow Tang to kill my husband's new fish.. he hasn't even seen it yet because he works til 12. Someone please help! :(