Just to provide my personal experience, I had to empty my tank to get a 4 stripe (black and white) Damsel, and the evil Blue Damsel. Should have been called 'Blue Devil Damsel'. I think my tank was just too small for them to be happy - just absolutely beat everything up. I think I did catch one once, but probably only after a month of trying. And I still had the heart to bring them back to the store (2-3 years later) to give it back to them for $5 store credit.