I bought 2 mini aussies from the same litter from which I later realized was a puppy mill. I have had health issues with the female as a puppy but they are both now healthy. When Radar(male) is excited, when they are either losoe in the fenced in backyard or in their 12' x 12' dog run, he becomes aggressive and will attack his sister. I seperate them as quickly as possible and they have never drawn blood but this is becoming more and more of a problem. I took them to training classes when they were about 6 months old and the training club used spiked training collars. and I now have 2 leash aggressive dogs towards other dogs. I walk them and he has improved. Guess the issue that I cannot deal with anymore is his aggression towards his sister and heis a fence runner/fighter. The minute he hers anyone when outside he goes wild barking and jumping at the fence.