You'd be fine with a 20 long for quite awhile. A 29 gallon is the suggested size for a pair, but a 20 long has the same vertical footprint, and con very rarely swims more then a foot above the bottom. But when the male gets large, a 20 long will be to tight; they like to stretch their legs, so to speak. For instance, I had my breeding pair in a 20 long, and my male was really frightened of me- he would hide whenever he seen me. I upgraded him to a 40 breeder, and it's like he's a different fish. He follows me around everywhere, and even jumps out of the water for a cichlid stick!