Tomato Clowns

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I have 2 tomato clowns and 2 cinnamon clowns in a 55 gallon tank who are doing great together, I would like to add another pair of clowns but every time I attempt to they attack the newcomers, I have attempted a pair of orange skunk clowns which failed, I have attempted a pair of tomato clowns which failed .I have a yellow tang which they do not bother which I will be eventually giving away. Any suggestions on how to add another pair ?