What is Salinity, why is it important, what role does it have to play in the marine aquarium and finally, how do I test it?
Salinity. Essentially how much salt there is in water to make it 'salt water' as we know it. Interestingly, there is a variable scale of salinity in the ocean depending on where you are in the world and the season if you're on the coastline. The salinity in the majority of the oceans is 35 PPT (parts per thousand), this does vary in some places like the famous Dead Sea which has a salinity of 280 PPT! It's so salty you float in the water and there is little to no marine life, hence the name 'Dead Sea'. The Red Sea has a salinity of 40 PPT owing to the rate of evaporation and because it's a channel of water which isn't easily replenished by the Indian Ocean. More broadly however, salinity varies across the globe in relation to the equator and the poles. It varies between 37 PPT and 32 PPT so it averages out at about 35 PPT which is one of the reasons why we shoot for this number. The other factor affecting salinity in the wild that I mentioned was the season along the coastline, the main cause being rainfall. If it rains heavily inland then freshwater will rush out to sea via rivers and run off thus reducing the salinity temporarily. This doesn't really matter so much for the regular hobbyist like you and I unless you're trying to breed fish because certain fish will often coincide their breeding season when the salinity drops as this often precedes a bloom in zooplankton for the fry to feed on, but as I said its not too important for the regular hobbyist, it's just an interesting tip bit of information.
So, average salinity is 35 PPT and that's what we aim for in our marine aquariums. Why is it important? Simply put, because the animals we are trying to keep and care for in our aquariums have evolved over a millennia to live and thrive in saltwater that averages this level of salinity. On a deeper biological level, the reason why saltwater fish exist and freshwater fish exist has a lot to do with osmosis. In saltwater, the water inside a saltwater fish will be drawn out through the tissue due to the process of osmosis, in order to replace this lost fluid a saltwater fish must drink a lot of water which has to be desalinated in a process called diffusion which primarily involves the kidneys. This whole process is known as 'osmoregulation'. Keeping fish at a lower salinity (hyposalinity) over a long period of time will stress this osmoregulation process, potentially causing permanent damage to the kidneys. Keeping fish at a higher salinity (hypersalinity) will also stress the osmoregulation process and overwhelm the fish causing a degradation in fish health over time.
It's worth noting at this time that hyposalinity is used to treat parasites on fish and for shipping fish from overseas suppliers. The dips are used over a very short period of time greatly reducing the long term risks of hyposalinity whilst it is also used for a short period of time when shipping in order to reduce the biological load on the fish, reducing stress and the effects of shipping. If a fish has been kept at anything other than 35 PPT for longer than a 'dip' it should be gently acclimated over time. Raising salinity by a maximum of 2 PPT a day is deemed a safe range for acclimation.
How do I test salinity?
There are two ways which I would recommend you test salinity in the marine aquarium, they are using a refractometer and a digital device (usually a probe). Probes will have their own controllers and methods of calibration according to the manufacturer. The most common means of testing salinity however is the trusty refractometer, here's how to use one;
1. Make sure you have a refractometer that is specifically for testing salt water in a marine aquarium. You can find refractometers online for other purposes such as testing beer but they sometimes don't have accurate measurements on them however so it's always best to go for the right tool for the right job. I would recommend the Red Sea Refractometer that is carried in store, it's fit for purpose and I use it for my reef at home.
2. Refractometers are often designed to compensate for a temperature difference between the refractometer itself and the water its testing. I find that they always work better if the temperature difference is minimised as much as possible by either warming it up in my hand for a short time or by leaving it next to the aquarium for quite a while to bring it up to room temperature.
3. Calibration. Even if you calibrated it the last time you used it, I would recommend calibrating it before every use. A small knock or atmospheric change could knock the calibration out of whack and when 1 PPT matters, it's important to make sure it is calibrated. Calibration is simple however so not to worry.
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a. Use a pipette to take a drop of RODI water (I use RODI water bottled for use in car batteries that way you know it's at absolute zero for calibration and free from contamination. Shops like Halfords regularly stock this) and drop it on to the measurement surface of the refractometer.
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b. Close the guard, and raise the refractometer to the light (natural sunlight works best but not necessary).
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c. Look through the eye piece to view the measurement reticule. If it is out of focus, twist the eye piece to bring it into focus.
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d. The optic will be split between blue and clear blocks. Where the two colours meet is the salinity reading.
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e. To calibrate, use the screwdriver on the screw atop of the refractometer and turn it slowly to bring the blue line to zero.
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f. Success! Your refractometer is now calibrated.
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4. It's important to clean off any RODI water from your refractometer now because the test sample of aquarium water is relatively small so any contamination will have a greater effect on the result. Use kitchen roll to make sure all RODI water is cleared up.
5. Once you've done that you can use a pipette (make sure you flush it out a few times to prevent contamination) and drop a few drops of aquarium water on the measurement surface. Close the guard and bring the refractometer to the light. Look through the eye piece and read the measurement where the blue line sits. This is your salinity level.
Hopefully this answers any questions you have about salinity in the marine aquarium, why it's important and how to test for it. If you get really stuck however then come into the store and see how we use the refractometer or alternatively get your water fully tested with our in-store testing service.
Also, if you'd like to read more about parameters in the marine aquarium then have a look at the Parameters 101 blog linked below.
CLICK HERE FOR PARAMETERS 101