With only 6 young discus in a 125 gal., your timid fish may have found itself on the bottom of the pecking order. The stress of being chased will drive a fish to stop eating and seek shelter away from the group, which will cause it to stunt, and become more susceptible to parasites. Note: I doubt that treating your tank for parasites will solve your problem. The reason is only one fish seems to be stressed enough to be overcome. Treatment will only stress this fish more. Barring a huge stressful disaster in your tank, it's unlikely any of the other fish will be affected at all. Only stressed and weakened discus usually fall to parasites. Carrying parasites is natural for discus and most other SA fish as well. They usually do no harm to the fish. Only when the fish is weakened by severe stress do they become a problem. I think people freak out and treat discus for parasites unnecessarily. They think since they have treated for parasites, their fish do not have them. After the treatment ends, the parasites return. They don't even know it, since the parasites are not harming the fish. The side effects of the treatments often stress the fish worse than the parasite. There is usually a runt in every smaller group of young discus. If you remove the runt, then the next weakest fish will become the runt. Only in tanks with adult fish does this never happen. You can minimize this in young ones by keeping only large groups of little ones together. This spreads the behavior out so that it does not become a problem. When I buy young discus, I usually get a group of at least 12 fish to grow out together. They do much better this way.
