Fish are open systems which is why water quality is so important

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 1^oC.

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 <cocktail02.htm>] 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.

Without osmoregulation fish will die: Osmoregulation is the control of the concentration of body fluids. If a fish is unable to regulate the effects of osmosis it will die. Clearly, osmoregulation is a vital function affecting all aspects of fish health.

Osmosis, diffusion and fish health: If we look at the structure of the gill or gut under a powerful microscope we would see the tissue that lines these structures, the epithelium, is very thin, usually only one cell thick. Such areas are rich in blood vessels with very thin walls. The combined thickness of epithelium and blood vessels is less than the size of a full stop. The structure and thinness of these areas means that the minute particles whizzing around in the water (and blood) can pass through the epithelium and blood vessels with relative ease; in fact in many cases it is as if there wasn't any barrier at all!

If the gill and gut surfaces were unfolded and laid out flat we would have a massive area, much greater than the combined surface area of the skin and scales. At least half of the exposed surface of a fish is permeable to small particles. It is this permeability that leads us to describe a fish as an open system, constantly exchanging particles between its inside and the surrounding water. Now, perhaps, it is possible to understand why changes in the surrounding water have such an important effect on fish.

Diff and 'Oz': The permeability of the gills and gut and, to a lesser extent, the skin, which are all in close contact with the water, leads to a further challenge for aquatic animals. Diffusion or osmosis alone would eventually result in the fluid of a fish's body being identical to that of the surrounding water, just as a sugar cube, left in a hot cup of tea, would eventually fully dissolve and be spread evenly throughout the tea (by diffusion). If the fluid within the fish were to be dissolved thoroughly with the water surrounding it we would, I'm afraid, have a dead fish! But the composition of the body fluids is obviously different to the pond water and has to be maintained that way if the fish is to remain alive and healthy. So how does a fish manage to do this? What is it that keeps a fish's body fluids stable and different to the surrounding water?

Constant urination: The body fluids of a freshwater contain more dissolved salts and ions than the surrounding water. As a result of this imbalance there is a constant influx of water into its body and a loss of salts and ions from the blood outwards. A similar effect occurs if you put a dried raisin or apricot into distilled water. There is a net influx of water molecules through the skin of the dried fruit and it swells up to look like a grape again. (Try it!)

The mechanism may seem complex but essentially a fish has to rid its body of excess inflowing water by constantly excreting a weak solution of urine. Fresh-water fish can urinate approximately 30 per cent of their body mass each day. Salts are removed from the urine before it is excreted (fish are not wasters, remember) and they are also actively taken up from the water by way of the gills in order to maintain internal salt levels. This constant active absorption of salts requires energy but is essential to the fish's survival, and anything that affects this vital function will have serious effects on the health or even survival of the fish.

The reverse situation exists in marine fish; the environment contains more dissolved salts and ions than the fish's body so there is a net movement of water out of the fish's body into the stronger sea-water. To replace this constant loss of water, marine fish drink sea-water and excrete the excess salts. Special cells in the gills called chloride excretory cells are involved in this process.

Clearly, any interference to the fish's osmoregulatory systems, either fresh water or marine, would quickly prove fatal! Fresh water fish would rapidly accumulate water (a typical sign seen in dropsy), while marine fish would dehydrate. Clearly, our health considerations must take aboard the importance of osmoregulation which can be affected by instances such as water hardness, ulcer disease and stress.

To summarize, we have seen that a fish, because it lives in water, has to adapt to two major challenges, namely: low levels of available oxygen and the need to contantly regulate and maintain the composition of its body fluids.

It is the interaction between the chemical composition of the pond water and the need to maintain constant internal conditions that results in water quality being so crucial to fish health. http://www.fishdoc.co.uk/water/osmoregulation.htm