The 'rearranging' thing works as dave said. Just make sure if you do have to rearrange you add the peaceful one in first as you would if you were just putting them in. Usually (key word) the fish who goes in second will accept the other one as being there first. A lot of it just depends on how aggressive the fish is. I've never had parrots so I don't know but I have read that they are pretty aggressive.