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Understanding that fish are
'open systems' and intimately involved with their environment is
fundamental to understanding why water quality is so important for fish
health.
It is largely accepted in the hobby of koi and other types  of
fish-keeping that the majority of health problems relate, directly or
indirectly, to water quality but it is not always understood why aquatic
animals are so dependent on their environment and how changing aquatic
conditions can affect their health.
To appreciate the complexities of life in water we must first try to
understand the special problems and challenges that an aquatic
environment presents. There are, as we shall see, many fundamental
differences between life on land and life in water, differences that can
lead to major misunderstandings about fish and their needs.
It is too easy to accept the popular view that fish are somehow
inferior to other animals and to see them as primitive, poorly evolved
creatures. The fact that they are regarded, inaccurately, as being
'cold-blooded' seems to support this view and the idea that they
were left behind by more highly evolved animals such as ourselves.
Regrettably, the common view of them being lesser animals often leads to
them being treated differently from other pet animals. Yet when we look
closely we see that the similarities between fish and other animals are
greater than the differences. The differences that do exist are mainly
as a result of adapting to life in water.
An understanding of aquatic health has to start with the relationship
between fish and their environment, and a basic understanding of how
they have adapted to the problems posed by life in water. Other pages
will explain that water, rather than being an innate substance, is
actually a complex chemical cocktail with some very strange properties.
It is important to understand that even though they are solid and
covered with skin and scales they are not isolated from the water they
live in. Because they are dependant on their immediate environment for
many vital functions they are '"open
systems' and as such affected by nearly every change in the surrounding
water. To complete the picture of the difficulties of an
aquatic existence we need to know a couple more facts about water.
A ready supply of oxygen is important to almost all animals,
including fish. Air contains about 21 per cent oxygen, while water can
contain only around 8 to 9 per cent at average summer temperatures.
Oxygen dissolves into water slowly, and dangerously low dissolved oxygen
levels can occur under certain circumstances. Also, water is 800 times
denser (or heavier) than air, so we can see that living in water
presents a considerable challenge for oxygen-breathing aquatic animals.
Comparing fish with higher animals, most body functions are similar
and, with one important exception, the differences are minor. In common
with other animals, fish use oxygen for metabolism, producing carbon
dioxide as a waste product. Nutritionally, their food needs are similar
to most other animals. They need protein, fats, carbohydrates, vitamins
and minerals, just as we do. They have the same major organs - heart,
liver, brain and kidneys etc. - and highly developed sensory organs for
smell and sight, and a complex central nervous system.
So we see that fish, apart from being ectothermic and living in
water, are essentially the same as most other animals. If we accept this
then we can begin to appreciate the problems faced by living in water.
Firstly, fish live in an environment where oxygen is only sparingly
available so they must use it frugally and efficiently. Secondly, they
have to cope with the effects of osmosis and diffusion and to do this
requires expenditure of valuable energy.
A breath of fresh air
Animals need a constant supply of oxygen which they use in
combination with food to provide energy. The availability of food and
oxygen will determine the amount of energy any animal can use.
Whether an animal is warm-blooded or cold-blooded generally depends
on the availability of energy sources, that is, food and oxygen. All
animals have to maintain a fairly constant body temperature or their
body functions can be seriously disturbed. For terrestrial animals the
ability to maintain a constant internal body temperature is important
because of the large fluctuations in ambient temperature that can occur.
Just imagine what it would be like if your body functions came to a
standstill every time the weather turned cold, you could go to the
shops and not return for weeks! Despite constant changes in
environmental temperature, most animals maintain a constant body
temperature by consuming relatively large amounts of food and oxygen.
Digestion and respiration produce energy and large amounts of heat to
maintain a constant body temperature. In fact, most of the food that we
eat is used just to keep our body temperature steady to within about 1oC.
The situation is different in an aquatic environment. Water has a
high resistance to temperature changes so they occur slowly. So, fish
are able to maintain a fairly constant body temperature without
expending a lot of energy - which is just as well because oxygen and
food are not so readily available.
Hopefully, we can now see that being ectothermic is not due to a lack
of evolutionary development but is a necessary adaptation to a specific
environment, in this case water. This adaptation means that the energy
and oxygen requirements of fish are substantially less than for a
warm-blooded (or endothermic) animal, such as ourselves. Fish are energy
conservationists - we are comparative wasters!
Fish make efficient use of meagre levels of available oxygen and
thereby obtain sufficient oxygen to lead active lives. As we all know,
fish breathe through their gills. Oxygen is removed from the water by
diffusion; however, uptake is significantly enhanced by the very
efficient structure and mechanism of the gills. Their large surface area
and counter-current blood flow enables gills to extract up to 80 per
cent of the available oxygen. Perhaps it is now
obvious that anything which even slightly adversely affects the gills
will seriously affect a fish's ability to survive.
When we look at a pond or tank full of fish it is easy to think they
are isolated from the water; after all, they have a skin, scales and
mucus, so at least appear to be waterproof. At a microscopic level we
find the opposite is true. As already mentioned, water is actually a
chemical cocktail [see page "The Final Cocktail]
of many dissolved substances with many added during its life in the fish
or koi pond. In water, many of these substances, solids, liquids and
gases, sub-divide into small particles; either molecules or ions.
These particles are very small: if we take a molecule of water for
example, 2-3 million of them would fit across a full-stop on this page!
In liquids and gases the particles move around at speed (faster in gases
than liquids), which means that our image of pond water has to be
further refined to be that of a chemical cocktail in which the minute
particles of which it is composed are in constant motion.
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