The banggaI cardinals are said that they can be aggressive towards one another if they are in too small of a space or if they don't mate up. If you have a good size tank then you could probably get away with a couple sets of mates but as far as "schooling" they do not do this well. If you put 3 in the tank and 2 of them happen to mate up then it is suggested that you take out the other one because the can stress him to death.