you changed the tank. therefor the discus are not used to it. they will need time to adjust to the new "pecking order". the new fish may be larger, mor mature, or more aggressive than the rest.
The best way to introduce new discus to a tank is to first quarenteen them and medicate if you see signs of illness, then when you are going to introduce them to your tank feed your fish and then rearrange the tank so there are no established territories and finally put the new fish in following normal acclimation procedure.
If it has been a short time I dont see how internal parasites could be a factor unless they were already present and the stress of the change of tank mates caused stress in the fish and that caused a foothold for the parasites to develope.
Anyway, if the tank you are talking about is the same tank and inhabitants as you used to have in your signature, you probably will not be too successful with the discus. There just isnt enough room and many fish in the tank.-I mean alot going on in a little space.->This is just my $.02, but I think almost every serious discus keeper, here and on simplydiscus.com will agree.