Just to follow up a little. The problem with feeding rainbows high-protein and fatty foods is that, although they appear to be thriving, they will soon develop health problems. I had difficulty keeping rainbows for more than a year or so before they would start dying off mysteriously. A change to a mainly algae and vegetable matter based diet changed things a lot. The fish looked better, were more active, and lived much longer. Frequent partial water changes also helped a lot. There's a little more on rainbowfish diet here:
Keeping Rainbow fish
Tony
Tony