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I added three smalls at the same time and two paired up. Chased the third one around until he was confined to the space by my overflow box. I offered him up for free on a forum and somebody saved him.
Try a different type of cardinal. I got 2 bangaai cardinals at one time and one died and I just didn't get another one and I had the other bangaai for about a year and decided to get a black spot cardinal because it was only $5 and they are doing good together.