Like Sena said, bettas like shallow areas. I love my 20 gallon long. I can easily raise up to 40 young bettas in it. For a sorority of 12 it should work great!
Who am I? Well I am a Junior Member of the International Betta Congress and a breeder of white HMPKs, red dragon HMPKs, black/red dragon HMPKs, metallic multi HMPKs, blue masked HMPKs, and now multi color HMs. Soon to add black HMs. I will be showing in New Breeder class in fall of 2012.