Did you post pics of the build and did you manage to sex the BanggaisFunny you should revive this post. I just got 10 banggai cardinals today. At what size would you say they are sexually mature. I'm going to be building your banggai breeder next week some time, will post full build pics.
You need to sex them first, they will start to fight once the first pair is sorted out. Have you got several tanks? You will need to separate the pair from the rest, if you have several tanks you may be lucky to get several pairs.