You might want to try Convicts. They breed in 10 gallons but a 20 is a dream. If you want aggression you got it. Mine paired off but i had another female and she was getting beat up so I put in a male and, The de-scaled him litterally not one scale left on his body. So he is still alive right? I put him in a breeder in hopes of survival and when I wake up the next morning do you know what I find? A female black convict beating up the injured one and the injerd one is twice the size!!!!! How is that agression for yah? They werent even ready to breed yet. Ohh and the de-scaled one died the next day in a livebearer tank from serious injuries( No scales, barley any finns, crooked tail, Bleeding jaw, and a bacterial infection from the wounds). And they are the hardiest fish avalible and the breed like rabbits.