Please follow Mike's suggestion to start with your source water check. While doing that please post full tank parameters.
Have you owned this setup for 2˝ years or did you get it from someone else and move it?
What type of filtrations are you using (Live Rock, canister or HOB
Describe the lighting units (type bulbs, total wattage, and age of the bulbs)
What are the current water parameters? Ammonia, nitrite, nitrate, pH, salinity, temp, calcium and alkalinity. Please post numbers of results.
What are the current tank inhabitants?