I think you could get away with 5 more if you have a good biofilter and keep an eye on parameters. The more cardinals you have the more you'll enjoy them. They really look cool in a larger group.Do you think I could add another cardinal tetra or 2? I am actually trying to get a female ram for my male to finish off the tank. Yeah, I have never really had much love for cardinals, sort of like that hot chick with no personality, "look at me, Im hot" lol.