36 Salinity
Description
This article is from the Beginning Fishkeeping FAQ, by Thomas Narten with numerous contributions by others.
36 Salinity
Salinity refers to the total amount of dissolved substances. Salinity
measurements count both GH and KH components as well as such other
substances as sodium. Knowing water's salinity becomes important in
salt water aquariums. In freshwater tanks, knowing pH, GH and KH
suffices.
Salinity is usually expressed in terms of its specific gravity, the
ratio of a solution's weight to weight of an equal volume of distilled
water. Because water expands when heated (changing its density), a
common reference temperature of 59F degrees is used. Salinity is
measured with a hydrometer, which is calibrated for use at a specific
temperature (e.g., 75F degrees is common).
One component of salinity that neither GH or KH includes is sodium.
Some freshwater fish tolerate (or even prefer) a small amount of salt
(it stimulates slime coat growth). Moreover, parasites (e.g., ick) do
not tolerate salt at all. Thus, salt in concentrations of (up to) 1
tablespoon per 5 gallons can actually help prevent and cure ick and
other parasitic infections.
On the other hand, some species of fish do not tolerate ANY salt well.
Scaleless fish (in general) and some Corydoras catfish are far more
sensitive to salt than most freshwater fish. Add salt only if you are
certain that all of your tank's inhabitants prefer it or can at least
tolerate it.
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