if you are going to use the term breed then that should be the only kind of fish in the tank. The 55 should be large enough. Its the other tanks you need to keep the fry in and cultivating food for the fry. You need to research breeding rainbows. Also fry food. There is a lot that goes into breeding. Fry rearing is the hardest. Time consuming. My wife says she should be a fish. So I could spend more time with her.