Need more info! How long has the tank been up and running? what are the measurement? Nitrate, Ammonia, salininty, etc. What has changed? What type of filtration? How long has the live rock been in? How long was the live rock cured? How much live rock and how big a tank? Fish only or reef? What has changed recently? New fish added? Any signs of disease, ick?
The more info the more we can help!
Bob