Flank sucking or picking as it has been called where a fish can be observed picking or cleaning off the side of another fish, which usually just lays there allowing this action, can be caused by a tiny lesion on the fish which becomes nutritionally attractive to other fish because of the haemorrhage.

These lesions may be caused from abrasions and knocks during play, or at those wild feeding times, breeding behaviour is also another cause for minor body damage.

Under-feeding your fish you cause them to be interested in weaker fish or chewing at each other - this of course is easy to prevent .. simple feed them ! 

A sore on the fish is not always caused by other fish sucking or picking on them, It may Indeed, may be that an new Ulcer may be the primary symptom of a parasitic attack and could cause a secondary bacterial infection - so make sure your water quality is good and then consider a diagnosis or treatment for what may be a parasite.