Are they most aggressive after feeding? That's a clue that the bully has at some point been starved or gone without food for too long a time. This can be confirmed if the discus exhibits too large of eyes for it's body size or a narrow forehead. That's a tough one and they seldom get over it, but there are other things to consider before you send them back to the fish store. Water quality can affect behavior dramatically, a lower pH around 6.4 and 50ppm soft water has a calming effect as do "black water" additives, softer lighting and plants in moderation. If you are feeding bloodworms or other frozen food in big pieces causing a feeding frenzy, try "dissolving" frozen food in a small dish with tank water being feeding. Fed that way the bully will have so much food floating around them there's nothing to fight about. You might want to turn off your filters during feeding if you do this. Alternatively you could put a screen in the tank to seperate the bully giving him a "time out" for a few days. Internal parasites can cause aggression because they irritate the fish, and conversely discus can stop eating and become meek when they have intestinal parasites. I mix fresh pressed garlic with frozen bloodworms to "naturally" keep internal parasites in check. The young and older discus don't seem to mind the garlic, in fact they willingly eat pieces of it. Just introduce a very small amount at first and gradually increase the proportion remembering a little garlic goes a long ways.
