If you could post a picture it would be helpful in trying to identify the problem. There are several treatments, in the mean time check your water parameters get a test kit if you don't have one ammonia, nitrite, nitrate ect. What type of filtration are you using? How often do you clean it? What temp is the tank set at? What size is the tank, what else is in there? When was the last water change and how much was it (%)? You could do a 50% water change after you test. Clean water is the best prevention.