I say the cardinals as well. I would recommend Rummynose Tetras though hands down. I have a 37 gallon with 6 cardinals, 7 rummy's, 8 white cloud mountain minnows, 2 bolivian rams, a clown pleco, a betta, and lots of ghost shrimp. As I get other tanks together, I will be moving the Cardinals to a tank where I can have a much larger school and get more rummys. Both the cardinals and rummys stay in the middle to the bottom of the tank. The rams never mess with them and the betta stays towards the top and I am lucky that he is not really aggressive to any of his tank mates.