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The importance of water quality to fish health 
Fish-keepers should not underestimate the importance of water quality
to the well-being of their fish. As beginners we are told "look
after the water and the fish will look after themselves" or
"fish keeping is all about water-keeping". These statements
sound grand but what do they mean?
Water quality not clarity
Fish-keepers know that water quality is one of the most important
aspects of fish health. The problems arise from our interpretation of
what that means. Some fish-keepers believe that water quality simply
means water clarity'. But clarity often has little to do with quality.
Indeed our idea of what constitutes good water can be very different to
what is ideal for fish! So let me come right out and say it is not
possible to tell good water by just looking - although some fish-keepers
believe they can.
Consider a mountain stream. Often such streams near their source can
be crystal-clear but devoid of life. Further downstream the water can be
green and often muddy - yet such waters are usually highly productive
and teeming with all forms of aquatic life. So, in natural conditions
the clearest water is not always best for aquatic life. However, it is
possible to have good quality water that is also clear.
Why is water so important?
Understanding the importance of water to fish requires some
understanding of water chemistry and of the way in which different water
conditions affect the health and well being of fish. It is often
difficult for terrestrial animals such as ourselves to understand how
crucial the relationship is between fish and their watery environment.
It is only when we understand this relationship that the importance of
water quality' (or water chemistry) becomes clear.
Fish are 90% water living in a watery environment - and they are not
isolated from the water that surrounds them. They have to be open
systems to allow for the exchange of substances between themselves and
the water; substances such as oxygen, carbon dioxide, salts and
wastes. All that separates the fluid inside a fish's body from the water
that surrounds it is a thin membrane. This membrane is semi-permeable
(meaning that some substances may pass through it but not others) and
allows an exchange of substances between the body fluids and the
surrounding water. When fluids are in close contact they tend to mix
together to form a uniform mixture (like milk in your tea). Therefore,
changes in the nature of the surrounding water will have an effect on
the body fluids of the fish.
Fish have evolved over millions of years in fairly stable conditions.
Therefore, in a natural environment, they do not need to adapt to rapid
or large fluctuations in the conditions of their surroundings. However,
in an enclosed system, such as a fish-pond, conditions can change
rapidly, leading to stressed fish.
Consider temperature, for example. In an open body of water the
temperature may vary between 0oC and 25oC but this
would normally occur over a twelve-month period. Contrarily, for a
land-based animal, such a temperature change could occur over a period
of hours without causing distress. Whilst
fish can adapt to changes in environmental conditions their ability to
do so is limited and usually restricted to a narrow range.
Rapid or large shifts outside their normal range can cause health
problems or, in the extreme, even death.
To appreciate the full extent of the relationship between fish and
the water they live in we need to 
understand some basic chemistry and a few important properties of water,
and how variations in water conditions (physical, chemical or
biological) can affect the body functions of fish. Water is the most
common substance on earth, covering 70% of the planets surface. Plants
and animals are mostly water - even the Archbishop of Canterbury, is 65%
water! Because of this abundance we often take it for granted and
overlook what a remarkable substance it is. Water is a major part of all
biological systems and there is almost no activity of a living organism
that can take place without it.
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