How long has the tank been set up, and are these Rainbow fish new to the tank how long?
Do you know about the nitrification cycle, if not check the AA article in my signature link. very full of information about tank and fish keeping in general and good to read!
What are your water parameters?
If you do not have a test kit, please do a series of water changes.
Save a sample 1/2-1 cup, to take to a store BEFORE the water changes for comparison.
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves