Reed,
Do you have a temperature monitoring device in your tank? What kind of testing have you done? Do you use RO or DI or filtered water? Certain times of year water has more phospates from Tap. Do you have animals eating them like butterflys, angels, crabs? My guess would be water quality. I would assume nitrates are high. I would do a particial water change.
Wait a about 4 hours and test again. Lots of times people do water changes and do not prep their water before putting it in the tank. Make sure to mix salt mix with powerhead and use a heater to keep it as close of temp as the tank you are adding to. I use my sump to add top off water. That way you are not adding extra stress to animals. Any time you put your hand in the tank and move a rock, coral, you are adding stress to their environment. You want to do as little a change as possible not to stress animals. I am sure you know all this. But sometimes people over look the basics such as temp, salinity, etc. I did this a few times. I am fool that soon learned. I hope you are not doing what I did. I wish you luck. Please let me know what you find out.
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