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Keeping SPS Part 2, Its Salty

  • Keeping SPS Part 2, Parameters.

    2. A) Salinity

    Next to filtration methods, the most discussed part of our hobby amongst aquarists is probably parameters. It is generally the first port of call when hobbyists experience problems with small polyp scleractinians, or for that matter any of their precious charges, and correctly so. Simply put, the ocean from which our corals originate has a specific composition of elements, each in a specific ratio to make up the largest concoction of salty soup on the planet. In order for us to keep corals successfully, we need to resemble this complex soup called seawater as closely as possible, or at least try.
    This rings especially true when we want to ensure that our corals achieve good growth and colour successfully over the long term. Even with the vast number of hobbyists now succeeding in keeping these corals alive, as opposed to 20 years ago, they still prove to be a challenge to most hobbyists, predominantly due to their sensitivity to the specific makeup of the salty soup in which they are kept.
    It goes without saying that we are dealing with salt water, and before we start looking at the major and minor constituents of the water, perhaps a closer look at the overall saltiness would be a good starting point.
    There are various terms used by hobbyist when referring to and measuring the “saltiness” of water, with specific gravity and salinity being the most prevalent. Simply put, both are ways of expressing the concentration of salt dissolved in solution, as these two elements are the largest “ingredients” in our salty soup. It is important to note that we cannot measure actual salinity directly, and regardless of the instrument used, we are measuring a secondary parameter and converting it to salinity.
    Unfortunately, these two terms are often confused by hobbyist, often to the potential detriment of our captive specimens. It is commonplace on the vast array of internet forums to find less experienced hobbyists using these terms interchangeably.
    Firstly, let us look at the term specific gravity, or SG.
    Before getting into the difference between specific gravity and salinity, it is important to differentiate between specific gravity and density.
    Simply put, density is expressed as a measure of the amount of mass per volume, whereas specific gravity is a measure of the density relative to the density of a reference substance. In our hobby, we use pure water as the reference substance to show the density of saltwater when compare to it.
    Simply put, SG is the comparison of the “density” of a sample of water when compared to the “density” of pure water at 4 Degrees Celsius (is the point at which pure water is at its most dense, although different organisations and fields use different temperature standards.)
    Herein lays the caveat with measuring SG, as it is entirely temperature dependant and the lower the temperature, the more dense a sample will be, even though the salt content of the water (weight for weight) does not change.
    It is measured either by using a swing arm hydrometer or by floating hydrometers and expressed as a value compared to the SG of pure water, which has a SG of 1.00.
    I will not go into detail on floating hydrometers, as they are seldom used by hobbyists today, most likely due to them being more difficult to read when floating in a tank with sufficient flow for SPS, or, when kept in a sump in an area of less flow, one needs to get down on all fours to read it.
    Slightly easier to read, and more popular in the hobby, are swing arm hydrometers. Unfortunately, the readings obtained by hobbyists are misinterpreted due to an incomplete understanding of exactly what specific gravity is and just how temperature dependant the reading is. To further complicate matters, as temperature increases, the specific gravity of the standard (pure water at 4 ®C, or 20 ®C in our hobby) decreases at a different rate than saltwater. Fortunately, good quality swing arm hydrometers factor this in and are calibrated at a specific temperature, usually from 15 ®C to 25 ®C. Again, unfortunately, not all hobbyists know this and therefor don’t ensure that the water they measure is at the same temperature as the calibration temperature of the hydrometer.
    To illustrate, when using a swing arm hydrometer calibrated at 15 C, and your tank water is at 26 C, a swing arm hydrometer will measure 1.023, when the actual SG is 1.026. As mentioned, we want to provide our corals with parameters as close to those found in their natural environment, which is an SG of 1.026 at 20 C. Without understanding the workings of a hydrometer, and not having conversion tables at hand, a hobbyist might interpret the reading of 1.023 as not salty enough and consequently add more salt. See figure one for an example of a swing arm hydrometer.
    Figure 1, and example of a swing arm hydrometer

    There are other considerations as pointed out by Randy Holmes Farley, such as a hydrometer changing its own density due to the temperature it is exposed to. Furthermore, the sample being tested can also change its density as a result of the room temperature being different to the tank temperature of the water being tested.
    Even though the above might sound daunting, and quite frankly, when one gets into the nitty gritty of temperature compensation and SG, it is, good quality swing arm hydrometers (when used properly) are accurate enough for most marine tanks. There are some other considerations for the hobbyist using a swing arm hydrometer, such as ensuring that the unit is properly rinsed in fresh water to avoid salt deposits which can affect the next reading and when doing a reading to ensure no air bubbles are trapped on the swing arm. Notwithstanding that, we are not just keeping marines, we are keeping rather temperamental and sensitive corals, which leaves us with little room for error. The old adage of nature knows best, really rings true here….

    To avoid the above head scratching, it is far easier to spend a bit more money and instead of measuring specific gravity, rather measure the refractive index in a device which does the temperature “conversions” and conversions from the measured parameter (refractive index) to salinity a lot more accurately and hassle free.
    Salinity is a measure of how much salt is dissolved in water (weight per weight) and is expressed as parts per thousand (ppt). It does however not identify exactly which salt is dissolved and in which quantities, but is merely an overall measurement of the dissolved salts.
    Figure 2 Shows a good quality hand held refractometer, which has been specifically calibrated for use in seawater and offers Automatic Temperature Compensation.

    The instrument used by hobbyists to obtain a salinity reading is called a refractometer. A refractometer works on the principle that when light waves pass through the boundary between two media with different densities (refractive indices), the light is refracted, or bent, due to its speed being altered. The refractive angle is determined by the different densities of the two media, as the more the light is slowed, the more it is refracted.
    The most abundant in the hobby is by far handheld refractometers, due to their accuracy, ease of use and price. They work as follows: A water sample is placed on a prism, with light passing through the water and the prism and as it does, the light is refracted according to the refractive index of the water. The refracted light is then channelled through lenses and onto a scale, reflecting salinity as ppt (often expressed as ‰) and or as SG. See figure 3 for an example of the scale showing both salinity and specific gravity values as found in a good quality hand held refractometer.
    Figure 3 shows the scale which is visible when looking into a refractometer.

    Remember, we cannot directly measure salinity, and a refractometer measures the refractive index of the solution, which is then converted to salinity on a scale which we read. As different salinities have different refractive indices, we are able to use very complicated mathematical formulas to express the refractive index to a corresponding salinity. Fortunately for us, the math is already done by the manufacturers and is “built in” to the scale we read.
    Again, as with hydrometers, temperature plays a major role as water expands when temperature rises and contracts when cooled, which has an effect on the refractive index and consequent salinity reading.
    Fortunately for us aquarists, decent quality handheld refractometers automatically adjust for this to give us a more accurate result. In short, this is achieved by a bi metallic strip, connected to the lenses. As temperatures rise and fall, the metallic strip expands and contracts, which moves the lenses to effectively cancel out the expanding and retracting of the water sample. Not all refractometers have this function and it is therefore imperative to ensure that when purchasing a refractometer, it has automatic temperature compensation functionality built in. This is indicated by the abbreviation ATC.
    As with everything in this hobby, there is always a “but” and a consideration, and the use of refractometers are no exception.
    A reality often ignored by hobbyists, or something most are simply not aware of, is that most handheld refractometers sold in the hobby are actually not calibrated / designed for use with seawater. Instead, they are calibrated to be used to obtain results for sodium chloride (NaCl).
    When giving a reading for seawater, these refractometers usually give a reading of 1.7ppt higher. This is however easily corrected by ensuring that if you want to keep your tank at 35ppt (which really is where you want to be to provide salinity closest to where your sps come from) then ensure that your water is adjusted to read 36.5ppt.
    There are some handheld refractometers which are calibrated for seawater, such as the D-D unit, featured in image 2.
    Another option, although a bit more costly, is a digital seawater refractometer, such as the Milwaukee MA887 Digital Salinity Refractometer with Automatic Temperature Compensation see figure 4.

    Figure 4, The MA887 digital refractometer by Milwaukee instruments.

    A refractometer does need periodic calibration, and when calibrating the unit, it is important to remember that calibration must be done at the correct calibration temperature, which is usually 20C. This refers to the equipment temperature and not the temperature of the water. Due to the small amount of liquid used, the water on the screen will take on the temperature of the equipment within a few seconds.
    Most suppliers recommend using distilled or reverse osmosis water for calibration. However, some hobbyists use reference samples which are closer to the value of the water we keep to calibrate. In depth discussion about calibration is however beyond the scope of this article, however, I do recommend you read an excellent article by Randy Holmes Farley called Refractometers and Salinity Measurement, available in the December 2006 article in the online magazine,

    Lastly, it is important to ensure that your refractometer is properly cleaned after each use, to avoid salt crystals being left behind on the glass prism, which will affect the next reading being taken by giving a false high reading.

    Another piece of equipment used by hobbyists to obtain a measure of salinity, is a conductivity meter. In previous years, conductivity meters have been slightly above the budget of most aquarists (let’s be honest, most of us would rather buy that stunning acropora). However, with the advent and growing popularity of aquarium computers, combined with the ease of use of a conductivity meter, there has been an increase in the amount of hobbyists using it.
    Figure 5, an example of a conductivity meter, designed to be used in seawater, the Hanna HI 98203.

    The principle operates on the measurement of electrical flow between electrodes at a specific distance from one another by introduction of an Alternating Current through a probe. The flow is measured in milliSiemens per centimetre (mS/cm), and via complicated mathematical formulas, an accurate conversion can be mad. As with specific gravity and refractive index, the conductivity is largely temperature dependant. Fortunately for us, most conductivity meters designed for aquarium use do the conversion and temperature compensation for us.
    It is important to note that not just any conductivity meter can be used, as there are many types available, all with different ranges in which they are able to operate. When buying a conductivity meter, it is important to buy one which includes the value (53 mS/cm) within its range.
    Lastly, conductivity probes do need occasional calibration. This is done by using reference solutions with a known conductivity. It is important to note that when buying calibration fluids for your conductivity probe, you buy the one which is closest to the range you intend your probe to operate in. Figure 5, shows a Hanna HI 98203 conductivity meter intended to measure conductivity in salt water.

    The subject of salinity is a rather in depth subject, and does involve a lot of very technical, scientific and mathematical considerations. I hope this article has assisted the reader in gaining more thorough understanding of the basic terms and equipment involved to effectively manage this parameter in our aquariums. As this article is specifically aimed at keeping sps, I have not covered subjects such as hypo salinity, or any deviation from the salinity most commonly encountered on the reefs from which our sps corals mostly originate. In all my years of keeping marines, I have not come across any evidence showing a better salinity to keep sps corals at than 35ppt, or an SG of 1.0264 at 25 ®C or a conductivity reading of 53 mS/cm. Once your water is at the correct salinity, it is of paramount importance to maintain it there constantly, but this is a topic for another article all together.
    I really suggest that readers take the time to read literature posted below to obtain an even better understanding.
    In closing, I’d like to express my sincere gratitude to Keith Baxter from 555 Aquariums / Nemo’s Janitor and Stuart Bertram from D-D The Aquarium Solution Ltd for their input and assistance. You have both been instrumental in the writing of this article and in me gaining a much better understanding of the concepts involved.


    Randy Holmes-Farley, Chemistry and the Aquarium: Specific Gravity: Oh How Complicated!, Advanced Aquarist Online Magazine, January 2002.

    Ronald Shimek, What are Natural Reef Salinities and Temperatures…Really…and Does It Matter? Aquarium Fronteirs – 1997.

    Julian Sprung and J.Charles Delbeek, The Reef Aquarium, Vol. 3: Science, Art, and Technology.2005 Two Little Fishies, Inc., d.b.a. Ricordea Publishing

    Randy Holmes Farley, Refractometers and Salinity Measurement. Reefkeeping.Com, December 2006.

    Randy Holmes Farley, Using Conductivity to Measure Salinity. The

    Irie Ivan

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