- Jul 1, 2015
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What is your ideal salinity. My tank is at 79 degrees F and has a specific gravity of 1.022. I read in ppt that that equates 31.6 ppt salinity. What is the ideal range you guys have success with??
There are a variety of different ways to measure and report salinity, including conductivity probes, refractometers, and hydrometers. They typically report values for specific gravity (which has no units) or salinity (in units of ppt or parts per thousand, roughly corresponding to the number of grams of dry salt in 1 kg of the water), although conductivity (in units of mS/cm, milliSiemens per centimeter) is sometimes used.
Somewhat surprisingly, aquarists do not always use units that naturally follow from their measurement technique (specific gravity for hydrometers, refractive index for refractometers, and conductivity for conductivity probes) but rather use the units interchangeably.
For reference, natural ocean water has an average salinity of about 35 ppt, corresponding to a specific gravity of about 1.0264 and a conductivity of 53 mS/cm. It often ranges from 34-36 ppt over reefs, but can be higher or lower locally for various reasons such as land run off of fresh water, or evaporation from a lagoon.
As far as I know, there is little real evidence that keeping a coral reef aquarium at anything other than natural levels is preferable. It appears to be common practice to keep marine fish, and in many cases reef aquaria, at somewhat lower than natural salinity levels. This practice stems, at least in part, from the belief that fish are less stressed at reduced salinity. I have no idea if that is true or not, but I've not seen evidence that it is true. Substantial misunderstandings have also arisen in the past among aquarists as to how specific gravity really relates to salinity and density, especially considering temperature effects. For example, the density of seawater is less than the specific gravity, and measurements with glass hydrometers may require temperature correction, but newer devices do not need the aquarist to make corrections. Consequently, older salinity or "specific gravity" recommendations may not actually be referring to the same measurements that aquarists make today, even if the recommended numbers have been handed down
My recommendation is to maintain salinity at a natural level. If the organisms in the aquarium are from brackish environments with lower salinity, or from the Red Sea with higher salinity, selecting something other than 35 ppt may make good sense. Otherwise, I suggest targeting a target salinity of 35 ppt (specific gravity = 1.0264; conductivity = 53 mS/cm).
I have used hydrometers, refractometers and now have the salinity probe which measures conductivity. This probe is by far the best.
Hydrometers are iffy and cannot be calibrated (except for doing the math to make adjustments). I find they degrade over time and must be kept clean.
The refractometer has worked well for me, but I find it must be calibrated every time I take it out. I also have always had difficulty seeing through eyepieces (microscopes, binoculars, etc.).
Using conductivity, which is new to me, is very easy and it holds its calibration well. I can leave the probe in the saltwater and add top off water and watch to reach an exact number without constant retesting.
Randy’s article above is excellent for learning the differences and making calibration solutions.
I used a hydrometer years ago. For my return to reefing i picked up an AquaMaxx refractometer. I like it but I recalibrate every use. I just picked up a used Pinpoint Salinity Monitor and i am loving it. Whatever you choose, calibrate and test.