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 !