As far as we can tell, D-D’s Seawater refractometer is perhaps the ever such device that is scaled and calibrated for use with seawater. “That’s crazy talk” you say as you peer upon your aquarium refractometer which was likely designed for use with saltwater or brine, a solution consisting solely of sodium chloride (NaCl). By comparison, although seawater contains a majority of sodium chloride, it also includes a significant portion of magnesium, calcium, carbonates and all of the major, minor and trace elements we have come to know and love and which impart on seawater a significantly different refractive index than a pure brine solution. The difference between saltwater and seawater may be small but in the neighborhood of where we measure aquarium salt levels, where a proper seawater refractometer measures 29 ppt (1.022 s.g.), the average saltwater refractometer is telling you your salt level is at 32 ppt (1.024 s.g.). So you may think that your salt mix is low in calcium and alkalinity but in fact, you just need to add more salt mix to get your water to a proper salinity, silly.
If you currently own a refractometer, you can go ahead and recalibrate it using a seawater reference solution of a known salinity but there’s something inherently unfair about having to “fix” an expensive testing device which should do the right job in the first place. What makes the D-D seawater refractometer different is that the scale is also designed for seawater usage so you can calibrate the refractometer with pure freshwater at 20C and the rest of the scale will fall right in line with expected seawater refraction values. We must admit it was quite a jar to go around to all of our tanks with our Sybon and the D-D refractometers to find out that all of our tanks were actually at much lower salinities than we thought, explaining away a whole mess of issues we’ve had in the past with running hypo in a fish or reef, or issues with calcium and alkalinity from certain salt mixes. There’s a whole lot more we could explicate about the re[purcussions of this, but for now we’ll just direct you to a nice article on salinity and it’s measurement by Stuart Bertram of D-D the aquarium solution. The D-D Seawater Refractometer costs about the same as other refractometers and it is available from dealers of D-D Aquarium Solution products.