When i first got my exodons i could only find 5 (in the UK they are quite rare) and within the first week one had been eaten by the others, luckily soon after i found another 3 in a store and since boosting the shoal size to 7 havent had any problems. I keep mine in a 25g tank with pleanty of hiding places at the sides and bottom for the fish to take refuge in should any of the nipping start going too far.
These fish are not for the faint hearted, their whole existance revolves around fighting and biting each other and anything they come across.