Salinity & Specific Gravity

I have written a computer program called Brack Calc that convert between salinity, specific gravity, and salt concentration. You can download Macintosh and Windows versions of this program here.

Sea salt is more than just sodium chloride, it also contains calcium carbonate and bicarbonate, various metal salts, and small amounts of phosphates, sulphates and nitrates. For this reason adding table (cooking) salt is not an adequate substitute. Neither are the aquarium tonic salts sold for community tanks. The best way to make brackish water is to use proper seasalt (marine mix) sold for reef aquaria.

If you have a marine aquarium, good quality water from the marine aquarium can be used in the brackish water aquarium. To make half-strength seawater for example, you would mix old marine aquarium water with fresh tap water in equal amounts. Always remember to add dechlorinator. The ‘old’ water from the marine aquarium needs to be of reasonably high quality, so check the nitrate level to make sure that it is not excessively high. Nitrate levels up to 50 mg/l will be tolerated by most brackish water fish without complaint.

Salinity is a measurement of how much salt is dissolved in the water. It is difficult to measure, so most aquarists prefer to describe the saltiness of the water as a density. The density of a substance is how heavy (how much mass it has) per unit of volume. Pure water has a density of 1.000, i.e. one litre of water weighs one kilogramme. Seawater is denser as it has salt dissolved in it. Typically seawater has a density of around 1.025, which means one litre of seawater weighs 1.025 kilo grammes.

Hydrometers and refractometers

To measure the density of the water you will need a hydrometer, a device that measures the density of a liquid. Most hydrometers sold for aquarists are calibrated to be accurate at around 20 to 25° C. Swing arm hydrometers are commonest. These are small plastic devices, which are filled with the water to be measured. In doing so an arm moves inside the device which points at a scale, from which the density can be read. Cheaper, but less often seen, are floating hydrometers. These look a bit like glass thermometers with a weight at the base and a scale running up the glass. These are also sold with brewer’s kits for people who make wine and beer. So long as the scale runs from 1.000 to 1.030 or so then they are fine. Floating hydrometers are placed in the tank or bucket of water to be tested and left for a moment. When the device settles, the surface of the water will cover the scale up to the density which can be read off.

The density of water required varies depending on the fishes kept. Fishes from the fresher end of the range (like rainbowfish and kribensis) enjoy a density of less than 1.005, as do most freshwater plants. Fishes which live mostly in from the sea, like dog face puffers and batfishes, prefer a salinity of over 1.010. But most brackish water fishes can tolerate the entire range: from 1.000 to 1.025. These include such fish as sailfin and black mollies, sleeper gobies, monos, scats, archerfish, and shark catfishes.

If you can’t get a floating hydrometer which shows the entire range the alternative is to use a swing arm hydrometer and dilute full strength seawater. For example, if you want one-tenth seawater, or about 1.002 to 1.003, you would make full strength (1.025) seawater and then dilute it with freshwater in the ratio one part salt to nine parts fresh. One half seawater, about 1.012, would be made by mixing one part salt water to one part fresh.

What about the hand-held refractometers now widely sold online for around $50? On paper at least, these appear to be more accurate than hydrometers. In practise though, consumer-grade refractometers are not comparable with lab instruments. A hand-held scientific-grade refractometer would cost aroudn $1000, and desktop instruments even more. Are the cheaper, consumer-grade refractometers even useful? Yes, they work at least as well as good quality hydrometers. Both refractometers and hydrometers will only be accurate if maintained and used correctly. But it is far from clear that refractometers are actually better than hydrometers, as this discussion over a Reef Central illustrates.

Can you measure out the salt in teaspoons or tablespoons?

Measurements like one teaspoon per litre are sometimes mentioned. One teaspoon of salt weighs about 6 grammes, so very roughly, you can estimate the amount of salt using teaspoons. Water at SG 1.005 for example contains 9 grammes of salt per litre, so for a 10 litre bucket, you’d add 90 grammes of salt, or 90/6 = 15 teaspoons.

Making up your water

However you measure your salt, it is important never to add the salt directly to the aquarium. Not all the salts dissolve at the same rate, and some can damage your fish when undissolved. Always make the salt water up separately, following the instructions on the package carefully. Allow the salts to fully dissolve, there should be no residue at the bottom of the container. Stirring helps, but best of all use an airstone to bubble the water for twenty minutes or so.

Salt water is highly corrosive. Most metals corrode quickly even when splashed by salt water. It is therefore important to make sure that no metal objects are used inside the aquarium, and metal hoods and stands should be avoided.