withdrawn yellow and green stars

[commodore]

My yellow polyps and green star polyps seem to have withdrawn their polyps. Watr cheistry tests out good-ph,calcium,alkalinity,salinity,etc.Magnesium is low(1160) but adding daily to raise it. 4X 39 watt Tek fixture with Tek reflectors. Bulbs may be a bit old and replacements on the way.
Any ideas of why the polyps have withdrawn? Could the light be too bright, or the bulbs to old? Thanks

 

[jim t]

That a tough one. Those corals grow like crazy. I have had some problems with my tanks but those 2 corals are the toughest ones I have. they should be fine if your water quality is ok.