I have a pair of males, think they are siblings from their identical markings, who seem content to just hang together. Calvus are normally not territorial unless breeding, so it may be that you've already reached maximum stocking. They are best kept in pairs according to Aqualex (also 400 litres minimum, but they often state high numbers!), so I doubt that introducing more would achieve much except perhaps to worsen the aggression.
Do your calvus have caves, UKSouth? I know they normally just like to hang around observing but mine seem to like having a cave to retire to as and when.