Are these young discus? There is always a bully among a group of juvenile discus, IME. No matter how many you have, one will always throw his weight around. Adults, from what I've read and seen with my one adult, calm down.
First, we need your tank specs...
-size of your discus
-Temp...should be at least 84F
-Ammonia, nitrite, nitrate readings... they should be: ammonia- oppm, nitrite-0 ppm, nitrate- 10ppm or under.
-size
-other fish besides discus
-Is your tank non-planted, planted, or bare-bottom?
Discus should only be kept in pairs, for the most part, if they have actually formed a bond with each other... and your discus, if young, aren't old enough to do so.
Young discus should also be kept in groups of 6-8 fish; this keeps the bully from over stressing one or two fish.
It also sounds like you didn't quarentine the new fish; around 6 weeks for new discus that will be going into an established tank that already has discus in it. It is very important to do so... and sadly, many overlook it. I once overlooked it and almost lost all my fish because of it; now I wouldn't even think about skipping the process.
HTH
