Kribs can be very territorial and normally its best to only keep 2 in a tank unless you overstock by having a cichlid tank. 2 males or females will fight quite a lot of the time for territory and so its always best to give them lots of room so they have enough room to retreat to. It might also help if you get 1 male and 1 female as this will lower the rate of aggression. Personally I wouldn't keep any more than 2. But thats just me, I'm use to having a small tank. Hope that helps a little bit.