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The Importance of Ideal Aquarium Water Chemistry

This quick primer will help you easily understand the basics of water chemistry parameters

By Ed Mackey, MS, Ph.D

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Fish, such as this Nothobranchius species, have osmoregulation systems that keep internal fluids and electrolytes balanced. Good aquarium conditions help the osmoregulatory system function without much stress on the fish's body.

Water makes up the world in which fish live. Pure water is formed from two molecules of hydrogen (H2) and one molecule of oxygen (0) combined to form H20. However, water does not remain pure for long. After gathering as condensation in the atmosphere, droplets of water form and begin to absorb impurities, pollutants and other atmospheric particles as the droplets fall back to Earth as rain. After returning to Earth, water is influenced by the soil, rocks, lakes and rivers into which it falls. Evaporation and dilution, biological processes and other activities all influence water's characteristics.

Because of the interaction of all these variables, water composition is different throughout the world. All types of aquatic environments (lakes, streams, marshes, swamps, rivers, delta areas and oceans) contain aquatic species that have evolved in those specific water conditions.
The chemical characteristics most important to aquatic life forms are: pH, temperature, salinity, hardness, dissolved oxygen and carbon dioxide content, and of course, pollutants present in the water soup. Understanding the water conditions needed for a particular fish species is paramount for success. This understanding can help you provide the best environment for raising and breeding your specimens.

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Fish are poikilothermic, meaning that their body temperature is that of the surrounding water. That doesn't mean fish can survive in any temperature; they have evolved to withstand certain temperature ranges.

Acid and Base Balance
The pH value is a simple measurement of whether a sample of water is acidic, alkaline or neutral. It is determined by the number of hydrogen ions present in the sample. The pH scale is a numerical scale ranging from 0 to 14, 7 being the middle and therefore considered a neutral pH. A sample of water in the liquid state consists of positively charged hydrogen ions (H+), along with negatively charged hydroxl ions (OH-). The amounts present of each - the hydroxyl or hydrogen ions - make the water either acidic or alkaline. Through the process of evolution, fish have adapted to different environments and different pH values. To maintain a healthy environment in the aquarium, monitor the level of pH.

Fish evolved in water that remains relatively stable in temperature. Natural bodies of water change temperature slowly due to water's high specific heat capacity, and any changes in temperature that occur in a rapid sense happen because of a large amount of cold water from snow or melting ice, or from effluence coming from factories or power plants.
Fish are essentially poikilothermic, meaning that they are cold-blooded and take on the temperature of the surrounding water. Fish can be divided into two broad categories: those that have evolved into warm-water species or those that have evolved into cool-water species (warmer than 75 degrees or cooler than 75 degrees Fahrenheit, respectively).

The ph Scale

If the number of hydroxyl ions equals the number of hydrogen ions, then the sample of water would be considered a neutral pH (7). If, however, the hydrogen ions (H+) outnumber the hydroxyl ions (OH-), the sample of water becomes more acidic, and the pH decreases below the pH of 7. The greater the number of hydrogen ions, the lower the pH becomes. Conversely, the greater the number of hydroxyl ions in the sample, the more alkaline the sample becomes, and the pH begins to climb above 7.

It is important to note that the pH scale is a logarithmic scale, meaning that each number change is equivalent to a tenfold increase or decrease in the hydrogen ion concentration. This information is especially important when dealing with aquatic specimens in a closed environment. A small change in pH value reflects a large change in acidity or alkalinity. When substances are added or dissolved into water, those substances may add to the ion concentration, making the water sample more acidic or alkaline. A common example of this is the addition of carbon dioxide (CO2) as a plant nutrient. The addition of carbon dioxide lowers the pH as carbon dioxide combines with molecules of water to form carbonic acid (CO2+ H2O = H2CO3). This change adds to the acid.

Maintenance of the aquarium water at a proper temperature is paramount to good aquatic husbandry. Changes in water temperature affect metabolic rates and respiration due to the fact that warmer water holds less oxygen. Sudden changes in temperature may also affect growth, larval rearing, and incidence of disease or physiologic functioning. The fish's osmoregulation system, which balances internal fluids and electrolytes, is impacted by thermal changes in the water environment. Adding aquarium salt is one method of decreasing the stress. The ions in the aquarium salt help to decrease the osmoregulatory load in the fish.

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aquarium water chemistry
Fish species have evolved in all types of aquatic environments, so they have adapted to differing water chemistries. The important chemical characteristics that can vary between environments include pH, temperature, salinity, hardness, oxygen and carbon dioxide.

Water Hardness
Water is considered the "universal solvent" by scientists because many substances dissolve in water. Natural water contains essentially eight ions: sulphates, carbonates, chlorides, bicarbonates, sodium, calcium, magnesium and potassium in varying amounts. Other dissolved substances include nitrate, silicates, iodine, copper, phosphates, etc. The amounts of these dissolved substances determine hardness and the salinity of the water.

Hardness basically refers to the amount of metallic ions that are present in the sample, the main ions being calcium and magnesium. Hardness is usually referred to as total hardness (GH) or temporary hardness (aka carbonate hardness or KH). Temporary hardness (KH) is the major portion of total hardness and can be removed through precipitation if the sample of water is boiled; however, permanent hardness will remain in solution. Carbonate hardness is caused mainly by bicarbonate ions in the water sample, and this plays a large part in alkalinity. Measuring total hardness (GH) and carbonate hardness (KH) is done via test kits or electronic metering.

Osmoregulation in the fish body is affected by water hardness. The greater the amount of dissolved salts in the water sample, the higher the hardness, and the lower the osmotic difference between the internal environment in the fish and the external water in which the fish lives. This creates a lower demand on the fish's osmoregulatory system. Conversely, softer water creates a larger demand, and fish osmoregulation requires more energy. Fish have evolved to thrive in different levels of water hardness, so keep fish that thrive in the type of water you have, rather than attempting to change the water to match the fish.

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Barclaya Logifolio
Plants, such as this Barclaya longifolia, benefit from added carbon dioxide. But because CO2 can build up to dangerous levels in planted tanks with added carbon dioxide, it is important to monitor this chemical component in the aquarium's water..
Dissolved Oxygen
Dissolved oxygen in the water sample is determined by the amount of consumption and replenishment. Consumption occurs through oxidative processes in the aquatic environment and through respiration. Replenishment occurs through diffusion between the liquid-atmosphere interface and photosynthesis from plant life and many algae.

Oxygen is readily depleted in a sample by aerobic bacterial explosions that consume large amounts of oxygen. This occurs frequently in water samples that have a large organic load, thus requiring large amounts of aerobic bacteria for the breaking down of wastes. Algal blooms, on the other hand, have been shown to increase oxygen production through photosynthesis on bright days. High oxygen levels may cause oxidation damage to the delicate gills of the fish contained in the water. Fish, once again, have evolved to thrive in certain optimum ranges of oxygen saturation. If the oxygen saturation falls below that level, the health of the fish begins to decline.

Carbon Dioxide
Carbon dioxide is one of several dissolved gases in water. Carbon dioxide levels in the water are decreased by aeration and turbulence at the atmosphere-liquid interface, as well as plant growth.
Carbon dioxide has become increasingly utilized in aquariums in recent years as aquatic plantkeeping has proliferated. The use of carbon dioxide for aquarium plants is quite popular. Excess carbon dioxide in harder water samples is usually controlled by carbonate buffers. However, in softwater samples, there is a lack of carbonate buffers, and carbon dioxide levels can build up to dangerous levels for fish.
Monitoring water chemistry parameters is crucial to successful fishkeeping. You can monitor your water with simple test kits or elaborate electronic monitoring and controls for pH, hardness, carbon dioxide injection and carbonate injection. In any case, it is wise to monitor the status of water chemistry. Keep a log so you can track any short-term or long-term changes. AFI

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