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.
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.