Apologies for the length. Check out the TL;DR at the bottom if you're in a hurry. 
I've had my saltwater tank for ten years now, mostly as an SPS-dominated reef and have had generally good success. Then, about two years ago I had a crash that wiped out all my SPS colonies. I attributed the cause at the time to a nitrate spike causing a slowdown in the coral growth, leading to an alkalinity spike. The only things that survived were a green w/ purple tip branching LPS and a bubble coral - both among the first corals I got.The last 2-3 years, I'd started a really bad death streak of new fish in my QT as well. I would say maybe 9 of 10 wouldn't make it out of QT. They would be fine for the first week or so, then die. I QT mainly to facilitate salinity acclimation since our LFSes always keep the fish in such low salinities compared to the 1.025 in my DT, and raising 0.001 SG per day led to QT of about 1-2 weeks.About 6 months ago, my DT?stock was down to a yellow tang and a maroon clown. And my QT death streak continued. Whenever I get new fish I always check the parameters in the bag water and match my QT water to it, and all LFSes I went to (Neptune's Cove when it was around, Exotic Aquatics, and DNA) always had their water in the low teens, some even as low as 1.010, so acclimating up to 1.025 was a 2-week process of daily water changes. Then my maroon clown passed away about 3 weeks ago. With a basically empty tank and with me as the QT grim reaper, I was ready to throw in the towel and tear my tank?down.I really started to mistrust my LFSes and was convinced their fish being at hyposaline levels weren't able to survive such a drastic acclimation, no matter how slowly I took it. Moreover, my QT process, no matter how meticulous must really suck. LiveAquaria is well known for keeping their water at decent levels (1.018 for LiveAquaria, and 1.024 for Divers Den) so I figured I would try them out. I had never ordered livestock from online before, but I figured this would be a good option to shorten the salinity acclimation. I ordered a Kole Tang and a Snowflake Clown and when they came in (6 hours delayed, btw, due to bad weather) I tested the bag water - 1.010!!! WTF?!?!?It was at that time I started to think - what if my refractometer had been off this whole time because of bad calibration fluid? I have two bottles of calibration fluid - a Pinpoint one, and another of homemade fluid I matched with the Pinpoint one many years back. I've had my bottle of Pinpoint for almost 9 years (red flag). I did a quick search to see if calibration fluid went bad and consensus seemed to be that it didn't, as long as the cap is always kept on tight, and the bottle stored in a cool, dark place which both of mine have always been.The following morning I went to DNA with my refractometer and both bottles of calibration fluid to do a sanity check. They checked both my bottles with their refractometer (calibrated with tap water) and they came back at 1.035 and 1.037, respectively!!! Bingo. I just found my problem and it was years in the making. Because of my bogus salinity calibration, my refractometer was reading about 10 points of?SG too low, causing me to set my DT and QT at too high salinities. I rushed home and made some DIY calibration fluid based on RHF's directions, then checked my DT. It was at 1.032! I presume, my fish survived as long as they did because the salinity climbed up very slowly, over several years. My SPS, on the other hand, probably just couldn't take it anymore and started to give out due to the gravely high salinity. Their deaths then caused an alkalinity spike.My string of QT deaths must have been caused by me acclimating newcomers?way past normal salinity ranges, at which point?they just died. In my estimation, the fish must have come in from the LFSes around 1.020 (1.010 based on my bogus refractometer), and I would acclimate them to 1.025 which in reality was closer to 1.035!!! Even acclimating at 1 point SG per day, very deadly.I have since gotten some new calibration fluid (Two Little Fishies brand this time) - doubled-checked against my DIY RHF fluid and all is good. The Kole Tang and clown I got from LiveAquaria are acclimating in QT nicely. I've dropped my DT salinity down to 1.025 over the last week, and all appears well. This has re-energized my love for this hobby. I now will make it a habit to toss my calibration fluid every year, no matter how much I have left.TL;DR - my 9 year old salinity calibration fluid became more concentrated over the years, causing my refractometer to read SG about 10 points lower than in reality. This caused me to chase a much higher salinity in my DT, and acclimate new fish in my QT way past normal ranges, causing a string of new fish deaths, and a wipeout of the SPS colonies in my DT. I now have new calibration fluid and all is well!
I've had my saltwater tank for ten years now, mostly as an SPS-dominated reef and have had generally good success. Then, about two years ago I had a crash that wiped out all my SPS colonies. I attributed the cause at the time to a nitrate spike causing a slowdown in the coral growth, leading to an alkalinity spike. The only things that survived were a green w/ purple tip branching LPS and a bubble coral - both among the first corals I got.The last 2-3 years, I'd started a really bad death streak of new fish in my QT as well. I would say maybe 9 of 10 wouldn't make it out of QT. They would be fine for the first week or so, then die. I QT mainly to facilitate salinity acclimation since our LFSes always keep the fish in such low salinities compared to the 1.025 in my DT, and raising 0.001 SG per day led to QT of about 1-2 weeks.About 6 months ago, my DT?stock was down to a yellow tang and a maroon clown. And my QT death streak continued. Whenever I get new fish I always check the parameters in the bag water and match my QT water to it, and all LFSes I went to (Neptune's Cove when it was around, Exotic Aquatics, and DNA) always had their water in the low teens, some even as low as 1.010, so acclimating up to 1.025 was a 2-week process of daily water changes. Then my maroon clown passed away about 3 weeks ago. With a basically empty tank and with me as the QT grim reaper, I was ready to throw in the towel and tear my tank?down.I really started to mistrust my LFSes and was convinced their fish being at hyposaline levels weren't able to survive such a drastic acclimation, no matter how slowly I took it. Moreover, my QT process, no matter how meticulous must really suck. LiveAquaria is well known for keeping their water at decent levels (1.018 for LiveAquaria, and 1.024 for Divers Den) so I figured I would try them out. I had never ordered livestock from online before, but I figured this would be a good option to shorten the salinity acclimation. I ordered a Kole Tang and a Snowflake Clown and when they came in (6 hours delayed, btw, due to bad weather) I tested the bag water - 1.010!!! WTF?!?!?It was at that time I started to think - what if my refractometer had been off this whole time because of bad calibration fluid? I have two bottles of calibration fluid - a Pinpoint one, and another of homemade fluid I matched with the Pinpoint one many years back. I've had my bottle of Pinpoint for almost 9 years (red flag). I did a quick search to see if calibration fluid went bad and consensus seemed to be that it didn't, as long as the cap is always kept on tight, and the bottle stored in a cool, dark place which both of mine have always been.The following morning I went to DNA with my refractometer and both bottles of calibration fluid to do a sanity check. They checked both my bottles with their refractometer (calibrated with tap water) and they came back at 1.035 and 1.037, respectively!!! Bingo. I just found my problem and it was years in the making. Because of my bogus salinity calibration, my refractometer was reading about 10 points of?SG too low, causing me to set my DT and QT at too high salinities. I rushed home and made some DIY calibration fluid based on RHF's directions, then checked my DT. It was at 1.032! I presume, my fish survived as long as they did because the salinity climbed up very slowly, over several years. My SPS, on the other hand, probably just couldn't take it anymore and started to give out due to the gravely high salinity. Their deaths then caused an alkalinity spike.My string of QT deaths must have been caused by me acclimating newcomers?way past normal salinity ranges, at which point?they just died. In my estimation, the fish must have come in from the LFSes around 1.020 (1.010 based on my bogus refractometer), and I would acclimate them to 1.025 which in reality was closer to 1.035!!! Even acclimating at 1 point SG per day, very deadly.I have since gotten some new calibration fluid (Two Little Fishies brand this time) - doubled-checked against my DIY RHF fluid and all is good. The Kole Tang and clown I got from LiveAquaria are acclimating in QT nicely. I've dropped my DT salinity down to 1.025 over the last week, and all appears well. This has re-energized my love for this hobby. I now will make it a habit to toss my calibration fluid every year, no matter how much I have left.TL;DR - my 9 year old salinity calibration fluid became more concentrated over the years, causing my refractometer to read SG about 10 points lower than in reality. This caused me to chase a much higher salinity in my DT, and acclimate new fish in my QT way past normal ranges, causing a string of new fish deaths, and a wipeout of the SPS colonies in my DT. I now have new calibration fluid and all is well!