If you feed yo0ur rainbow a largely vegetable diet, you might be surprised how fast it will grow. I keep mine in a 120 gallon, 6 foot long tank and feed them a nice big portion of duckweed at least once a week. They love the stuff and 7 of them clear off the surface of a 6 foot tank in only a week. Right after that, it is not at all unusual to see breeding behavior. My rainbows are in a community tank of somewhat large, not giant, fish and they definitely can hold their own against most cichlids. As fish who prefer a vegetarian diet, they have teeth that demand the respect of any aggressors.
My rainbows.
Rainbow in breeding colors with fry
My rainbows.

Rainbow in breeding colors with fry
