Saltwater Aquarium Salinity

Water Quality

Saltwater Aquarium Salinity is a measure of the amount of salt in water and is measured in parts per million.  Specific gravity refers to determining the density of a substance by measuring how freshwater rises in a more dense liquid.  Fresh water at 4 C (39 F) has a specific gravity of one (1) and freshwater will float on top of a liquid with a reading in excess of this.  Specific gravity also varies with temperature.  To explain the relationship, saltwater at a temperature of 76 F and 31 parts per million of salt, has a specific gravity of 1.022.  This means it is 2.2% heavier than water.  Specific gravity can be easily measured with a hydrometer.  Fish only tansks should have a reading between 1.022 and 1.026.   Reef tanks should have reading of around 1.024 - 1.026.  In nature, oceans and lagoons have a specific gravity varying from 1.020 to 1.030.  Many fish can survive values above 1.030 for short periods.  Stability of the salinity is very important for the survival of your fish.  Since the oceans of the world are extremely large and interconnected, salinity requirements are nearly the same for all saltwater fish.

In order to set up a marine tank, proper water and salt are both needed:

The Water:  Using tap water is a possibility, but it comes with a risk that the chemical content is not known.  Although chlorine will dissipate after water stands for several days, other chemicals such as phosphates, nitrates and inappropriate calcium levels can cause contamination problems.  If you start with pure water, the process of mixing saltwater becomes much easier.  To obtain pure water, you can either buy distilled water or make your own.  To accomplish the latter, you will need a reverse osmosis (RO) filter.

RO filtering is the recommended approach to take for saltwater aquariums (but not for freshwater tanks).  The RO filter uses a membrane to remove chemical impurities and create purified water.  First a chamber removes heavy sediment.  Then a carbon filter removes chlorine and chloramine.  Thirdly, a thin membrane is used to remove bacteria, and other chemicals.  In order to remove nitrates and silicates, a fourth stage of deionization should also be included.  The four stage filters are referred to as RO/DI.  RO filters produce about 5 times as much discard water as they do purified water.  The resulting purified water is safer for fish, makes mixing salt easier and also reduces algae growth.  Maintenance of an RO filter is quite simple, as the filter only occasionally needs back flushing.  If RO filtering seems to be beyond your means, at least consider a drinking water filter, as these work on the same concept.

The Salt:  The next step is to add salt.  You can only add the salt to water in a tank that you are setting up for the first time.  After that, you will need to mix in a separate container.  Don't use table salt, as it has iodine in it.  Don't bother with rock salt either, as it won't have the right chemical composition.  Luckily, Marineland has come through to save the day with the development of their Instant Ocean premix.  It contains the proper amounts of salt, calcium and magnesium needed create the proper marine conditions.  With RO water and premixed salt, you can quickly establish proper saltwater.  Without the RO water, you'll be doing a lot of measuring of specific gravity, pH and calcium levels.  There are other products on the market that do the same thing as Instant Ocean, but none of these are close in popularity.  Remember that when changing water that you will need to add premixed saltwater, but when topping up a tank for evaporation you should use freshwater.  The reason is that evaporation leaves the salt behind and adding prepared saltwater will then increase the specific gravity too much.

Replacing Evaporated Water - When topping up a tank for evaporation you should use freshwater.  The reason is that evaporation leaves the salt behind and adding prepared saltwater would then increase the specific gravity too much.