I had the uncalibrated refractometer problem a couple months back too. I was using the solution and everything but still had problems. It was at 1.030 for who knows how long. I got a digital one to replace it and am loving how much quicker it is. When I brought it down it through off my alkalinity, calcium and magnesium dosing.So I tested po4 today, and it is .11, so 24 hours later, it has gone up .03 So, it now appears that I can suspend dosing p04. To me, this is good news, and ultimately means that sps should at least live. I will still continue to monitor daily to ensure this trend. Mushrooms are looking really good, so when open, usually po4 present, otherwise they tend to shutdown. I also recalibrated fracto, off by .03 sg. SG was 1.028 after calibration, so pulled saltwater and replaced with rodi, and adjusted to 1.025. Also changed out CO2 scrubber media.
2 issues I believe were causing death ro sps, main one, imo was po4 limited, 2nd, too much CO2. We shall see