How are things looking today Ray? If I notice that a coral is not looking its best the first thing I do is run the usual battery of tests...
Salinity - Test Weekly... Should be 27 to 35 ppt. or a specific gravity of 1.020 - 1.026. I maintain my reef tank at a salinity of 33-34 ppt. I recommend getting a refractometer to measure salinity instead of using a cheap hydrometer to measure specific gravity.
PH - Test weekly (or daily if having issues)... Shoot for a stable PH between 8.1 - 8.4.
Ammonia or NH3 - Test monthly... or if you notice something wrong. Should be zero.
Nitrites or NO2 - Test monthly... or if you notice something wrong. Should be zero.
Nitrates or NO3 - Test weekly... Should be less than (<) .2 ppm (parts per million). Basically, when this starts to rise it's time to do a water change.
Alkalinity or dKH - Test weekly... your alkalinity will determine the ability to maintain a stable PH. The lower your alkalinity the less stable your PH will be. It also helps keep your Ca levels stable. Shoot for 7-11 dKH.
Ca or Calcium - Test daily if you've just added corals to the system. Then weekly once you determine how much to dose to keep the value between 420-450 you can then test weekly to monitor depletion.
Mg or Magnesium - Test weekly... Mg is depleted by every living thing in your aquarium. It also has a direct correlation to keeping your Ca and dKH stable. Should be 1200-1400 ppm.
PO4 or Phosphates - Shoot for <0.05 ppm.You need "some" PO4 or your corals would starve. Any PO4 above that will degrade the coral's ability to utilize calcium and fuel nuisance algae.
If you think I came up with all of this after years of testing and experimentation you'd be mistaken....
This was my "go to" site when I first started in the reef hobby. Hope this helps.