Maybe that's the case with a trio, but I for one, have definitely seen cacatuoides flock breeding in bigger tanks. As long as each female has enough space to carve out her own territory, and the male has enough space to get away from all brooding females, it can definitely work. I wouldn't try it in anything smaller than a 30 or 40 breeder though. Again, making sure to have the sight lines broken up, and more hiding places than fish definitely helps. At the end of the day, a lot of it comes down to the personality of the fish as well. I've seen cacs who were total ***** cats, as well as males who would mercilessly and relentlessy hunt down and kill every female they were paired with.
