I agree with others that it is normal and will lessen with a larger group and with maturity. Dither fish will not have any affect on this discus behavior. For the most part, it is akin to sibling rivalry. I found that some chasing continues until they are about a year old. I guess maturity sets in and they are not so much worried about the other discus getting to the food first or similar fish thoughts. Sometimes there is a weaker discus and when they are targeted to be continually chased, they will not get adequate food and will be a bit stunted, or they may really take a serious turn and stop eating altogether...they give up. That 75G will easily support more discus if you'd like to add to the group
