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.
FW & SW
Favorite tank right now - 72G Bowfront Wadai Tetra, Cardinal Tetras, Ghost Glass Cat Kryptopterus bicirrhis and Clown Loach Cory haven
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article