Newer cockatiel causing old one to be aggressive toward mate
About a month ago, I adopted an older male cockatiel who was very neglected, and I already own 1 male and 1 female cockatiel.
Well, after I first introduced them all to each other, my original male cockatiel suddenly became VERY aggressive to my female cockatiel (they are usually best friends) and to me as well, yet he is very friendly to the new bird.
I don't understand what has happened, because I assumed if he would be mean to anyone, it would be to the new bird, yet that is the only one he is nice with.
Can you tell me what is going on and how to fix it so that my male cockatiel will resume being friendly with his mate and with me as well?
*Note* The original male and female are in a separate cage from the new male, if that makes any difference.