Why is your rainbow fish not eating?

Rainbow fish (Melanotaenia sp.) evolved in and are lovers of, the clean, gently flowing, warm (70F), creeks and rivers of Tropical Australia and New Guinea. If their tank water is not up to their requirements or if they are overcrowded they may well not feed, and sulk until they die. 1" of fish needs 1 gallon of water. A cycled filter is essential. 50% of their water changed weekly is also a good idea for most fish including Rainbows. My advice is for you to do a large water change.