I have a 4y/o pitbull/husky mix. He was raised on 15 acres. Me and my bf have recently been keeping him at our apartment a few days a week. When new people come in the apartment he gets aggressive, tries to bite, growls and sometimes nips. When at the house on 15 acres, he does not do that, he could care less about people coming. When outside at the apartment he is fine, no aggression at all. He ONLY does this at the apartment but with almost every guest. I dont know what to do. Please help me.