I recently got a new refractometer over the holidays, and immediately after my first water change my tank began having problems. Well, to be fair my wallet was what immediately had a problem being that those have gotten ridiculously expensive in my area, especially when combined with California's seemingly 3,000% sales tax these days. I couldn't figure out what was wrong, I mean, my water was at 1.025-1.026, my refractometer came with instructions that said to zero it out with distilled water, and they wouldn't give out bad information would they? Turns out yes, yes they would.
After doing a lot of research I came across endless debates online regarding zeroing a refractometer with ro/di water vs using calibration fluid. Both sides had good points: the scientific side said the only accurate measurement was with calibration fluid, and the general hobbyist side echoing what refractometer instructions say. Unfortunately it was the general hobbyist side that usually overwhelmed the debates, which made it seem like ro/di was the way to go. All I knew was that the refractometer was the only thing that had changed, and my tank was a disaster, so I decided it was time to test both sides of the argument.
I ordered this calibration fluid from Marine Depot, which was lab tested to make sure it was at nearly exactly 1.026. It wasn't easy seeing my tank go from bad to worse while I was waiting, fortunately though they're a local company and I had it the next day! I couldn't make it through my work day fast enough, my corals were dying and the cure might be sitting on my front porch.
When I finally got home I tore open the box and ran over to test the refractometer. As I peeked in my first reaction was, in all honesty, "wow, this calibration fluid is broken." It actually took me a few minutes to come to the realization they my calibration fluid was not broken. I'd figured the refractometer would be off by .001, maybe .002. Instead I was looking at a reading of 1.030. Was it true? Even after being zeroed out with distilled water like the instructions said, could it really have been reading .004 too high? Unfortunately, yes.
It's now a few weeks later and my tank is finally returning to normal. My chalice is gone, but the euphyllias are done their "who can peel the most flesh off the base" contest and the zoanthids are starting to look like their old selves again. I brought the calibration fluid in to work and both of our refractometers were reading .001 to .002 higher. I'm willing to bet that the vast majority of refractometers are reading higher than they should be, so do yourself a favor and spend the $10 on calibration fluid, chances are your corals will thank you.
If there's a moral in the story, it's that, as one of my favorite songs says, "universal truth isn't measured in mass appeal." So remember, if 90% of people say one thing is the way to go, it doesn't mean it's actually the best plan, especially when the 10% who are actually qualified to answer the question say otherwise.