Discus fish belong to the Cichlid genus and therefore should be kept in groups of at least 10 fish! (Exception: keeping a pair of fish/ breeding).
Typical behavior for this type of Cichlid is that, in smaller groups, often one fish will become dominant. This fish will then attack all other discus fish in the group, and often the group dynamic will result in the group preventing the weakest fish from feeding. The weakest fish will then become dark in colour, remain immobile in a corner, lose weight, will be excluded by the group and may die. If this happens, the group then turns on the next weakest fish and prevents it from feeding, and so on.
To prevent such behavior from continuing, we recommend that you increase the stocking density until the aggression in the group decreases. Experience has shown that discus fish behave peacefully in group sizes of 10 fish or more.
The described behavior is well-known in African Cichlids (e.g. Malavi, Tangajika). Discus fish are South American Cichlids with similar behavioural characteristics!