For a hobby that is founded on reproducing the needs of saltwater animals, reef and marine aquarists are woefully undereducated about what salinity actually is, and how to accurately measure it. So many problems with chemical and mineral imbalances in our aquariums can be attributed to simply not having enough salt in the water; low pH, low calcium and low alkalinity – all of these things can often be attributed to simply being a few points off in our measurement of aquarium seawater salinity.
The aquarium hobby’s gotten a little better in recent years with the proliferation of many better & cheaper refractometers but like any good tool, if you don’t know how to use it you can actually do more harm than good. For years reefers have relied on saltwater refractometers, designed for sodium chloride solutions (brine) at 20ºC/68ºF and using pure freshwater to calibrate at the 0 ppt setpoint.
Finally after decades of testing salinity in the dark we have a commercially made product from Two Little Fishes called AccuraSea which is designed to be a seawater reference & calibration solution. Regardless of what kind of salinity testing device you rely on, AccuraSea can you help determine how accurately you are measuring salinity, and it is more than enough solution to calibrate refractometers many dozens of times.
To “test” the solution designed to calibrate the testers we mixed up a batch of seawater of our own to the same 35ppt per thousand indicated by AccuraSea and first measured this solution using an ultra precise handmade hydrometer by Dupla Marin. Both the home-made seawater and AccuraSea solutions were brought to 25C/77F and we were not surprised to see the salinity readings using density and refractometry agreed.
We also tested the accuracy of using pure freshwater to calibrate three different hydrometers at the zero setpoint of 0ppt. It can be hard for the eye to discern between 1 and 2 points of salinity with even the best optics but thankfully the D-D, Red Sea and AquaMedic refractometers are among the easiest refractometers to look through.
This modern crop of marine aquarium refractometers are some of the best and most recently designed for aquarium use. When these seawater measurement tools are calibrated using pure freshwater, they deviate from 35ppt by about 1.5 to 3ppt – this means that even the best refractometers when calibrated at 0ppt can be off by more than 8% of the actual salinity, which means it’s 8% off in all of the constituents that make up salinity including calcium, carbonates, magnesium and everything else.
Now imagine you’re using a cheap, old or flawed refractometer and calibrating for 0ppt – if you’re not being very careful about your readings, sampling and measurements your salinity could be off by over 10%! That is why there is no excuse not to calibrate your salinity measurement tools at the actual salinity you are targeting and for all of us reefers that should be 35ppt, same as natural seawater.
We presume that when it is released, a 250ml bottle of AccuraSea should be within the budget of most reefers, especially considering how much headache it can save when balancing out other seawater parameters. Since there really is no other product like it widely available to reference our salinity testers, consider AccuraSea a mandatory addition to the reefers’ toolbox.