Purple firefish attacks mate!

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Hi, been a while. I'm hopeing someone can help. Stocking my 150 gal. upgrade. Have a bonded pair of Purple firefish. Added a clown (8 mo's later) One firefish has decided it likes the clownfish and is slowly killing it's mate! Got another clown. They're doing fine, though not yet bonded, but am at a complete loss as to what I can do.
Would SO apprectiate any help.