This article is from the Beginning Fishkeeping FAQ, by Thomas Narten with numerous contributions by others.
Water in nature is rarely pure in the ``distilled water'' sense; it
contains dissolved salts, buffers, nutrients, etc., with exact
concentrations dependent on local conditions. Fish (and plants) have
evolved over millions of years to the specific water conditions in
their native habitats and may be unable to survive in significantly
different environments.
Beginners (especially the lazy) should take the easy approach of
selecting fish whose needs match the qualities of their normal tap
water. Alternatively, an advanced (and energetic!) aquarist can change
the water characteristics to match the fish's needs, though doing so
is almost always more difficult than first appears. In either case,
you need to know enough about water chemistry to ensure that the water
in your tank has the right properties for the fish you are keeping.
Water has four measurable properties that are commonly used to
characterize its chemistry. They are pH, buffering capacity, general
hardness and salinity. In addition, there are several nutrients and
trace elements.
 
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