I had the same problem pairing maroon clownfish. Soon as it see the new fish it attacked it. I got a breeding net and put the new fish in it and put it as close to the other clown as possible, at first it tried attacking it thought the net but after a couple of days it stopped. When i let the new one out, there was a little bit of chasing but the new clown should start to shiver its whole body to show that its surrendering to the other one. Hope this helps. Also is the new clown noticeable smaller than the other? I found the smaller the new fish is, the better chance you've got of pairing.