Thanks! I will say that other than the API test kit I mentioned other testing kits are good (Hanna alk and phosphate, Red Sea calcium, aqua forest mag). TDS remains 0 on my RO system. I will try to gradually lower the temp a bit. Also, original post was wrong about phosphate: it’s been 0.05 to 0.1 (not 0.01). I think you may be on to something with the temp. I recently finished an ich fallow period and had the temp up at 80.6. Didn’t realize this could trigger the corals to expel the zooxanthellae.First you might want to invest in better testing equipment or at least go to a lfs with good testing equipment . Still waiting for a nitrate value . The Zero phosphate swing is concerning and also your temperature swing which can bleach corals . I personally won’t let my temp go over 78 , 80 to me is high and if it’s sudden could force the coral to expel its zooxanthellae which is bad for the coral . I would lower the temp , stabilize your parameters in correct margins and add targeted nutrients like fuel , Red Sea ab + etc . Wouldn’t hurt , but only after everything is stabilized and you’ve found and taken care of the problem that caused this downward decline to add live zooxanthellae in a broad spread and or target application . Also check your RO water , make sure your getting tds of zero before you make saltwater . If not add di to the system . I recently found high cloramines in my water which I had to add clorimine removing carbon blocks . So check your water supply and water out of your RO . Hope this and what others have stated helps !