SEA WATER

The main difference between fresh water and sea water is that the latter contains a great many salts which give it certain specific characteristics, and these must be understood by any aquarist who wishes to keep marine fish.

TEMPERATURE
The temperature of tropical sea water
varies little over the course of a day, or
even a year. Furthermore, marine fish are
generally more sensitive to abrupt
changes than freshwater fish. The temperature
in an aquarium must, therefore, be
fairly stable, remaining at around 25-26°C.

SALINITY
The most important salt found in sea
water is sodium chloride (NaCl), widely
used for domestic and culinary purposes,
but there are plenty more.
The salinity of water, i.e. the quantity of
salts in the water, is expressed in 0/00 or in
g/liter. The mean salinity of the Earth's
oceans is around 350/00, or approximately
35 g salts/liter.
Whatever its salinity, sea water boasts one
remarkable property: the proportion of
each element is constant.
Desalinated water does not therefore contain
less of one or more salts, but the
combination of salts is present in a lower
concentration.
The salinity of sea water varies according
to longitude. It is at its highest in open
seas in the tropics, it is lower near coasts
and after heavy rain, and it is at its lowest
near the poles (due to the influence of
melting snow).

DENSITY
In marine aquariums, it is not the salinity
of water which is measured, but the density
(often expressed as specific gravity,
S.G.), which can be calculated