As already stated, rainbowfish need to be in shoals, minimum of 6 for them to be happy. On reading another of your threads, unless you have missed out listing a tank, the largest tank you have is a 10g tank, this is far too small for the rainbow to live a long healthy life, turquoise rainbows (assuming thats what you have, M.Lacustris) require a minimum tank size of 30g, they should reach 4-5", preferably 55g for proper growth, so unless you can house it sufficiently and have the space to add 5 more, then it would be best for the fish if you returned it. Im sure thats not what you want to hear and if I dont have the full facts, I apologise in advance, just wanted to let you know.