If you can provide additional information about your set up, people can help you more and share more insight.
Are you having any other specific issues with the fish?
What other fish are in the tank?
What size is it?
What are your water parameters for Ammonia, Nitrite, Nitrate, pH, general hardness (GH), alkalinity (KH).
What is your maintenance routine? How much water do you change and how often?
What are you feeding the RF?