Water Quality Management

    In order to keep the health of any aquaculture system at an optimal level, certain water quality parameters must be monitored and controlled.  Some of these parameters that most directly affect the health of the system are pH and alkalinity, hardness, temperature, dissolved oxygen, and nutrients (such as nitrogen and phosphorus).  This page has been included to provide you with some background information for managing your system's water quality.


A pH meter

    pH is the scientific way to express the number of H+ ions in water.  As you most likely know, the pH of pure water is 7.  However, many sources of water have a low pH, meaning they are acidic.  These often range from 4.5-6.5, due to weak acids from the soil leaching into the water.  If the water becomes too acidic, it will not be able to support the growth of plants, fish, or invertebrates.  For this reason, water's pH should be monitored for declining pH.  This monitoring may be done in several ways, through titration, pH paper, or pH probes.  A low pH usually requires ongoing treatment, most commonly by adding agricultural lime (calcium carbonate, calcite, CaCO3), quick lime (calcium oxide, CaO), dolomite (CaMg (CO3)2), or builder's lime (Ca(OH)2).

pH paper test strips

    Water may become too alkaline (basic - a high pH) although this is much less common that low pH.  Fish can't live at pH levels above 11.  Ammonium Sulfate has been suggested to treat high pH, but this should be dome with caution since it tends to produce toxic by products.

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    All natural water is made up of more than just water, it also has many compounds in them, usually at least partially dissolved in the water.  The more dissolved material in the water, the harder the water is.  The softest water then is distilled water, which is nearly pure.  Hardness is generally used to express the total concentration of calcium and magnesium ions in fresh water, measured in parts per million (ppm) of calcium carbonate (CaCO3).  Soft water has 0-55 ppm, increasing to up to 211-500 ppm for very hard water.  Some calcium is necessary in aquaculture systems as it is needed for strengthening the bones of fish and the shells of crustaceans.

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Dissolved Oxygen (DO)

A dissolved oxygen meter

    All organisms grown in aquaculture need oxygen dissolved in the water in order to survive.  As you know, oxygen in produced by plants using sunlight, so if phytoplankton or plant populations are present in your culture, your oxygen levels will be lowest at night.  Fish and higher invertebrates are very efficient at removing oxygen from the water using their gills and they need to be, since water only holds a small percentage of the 20% of oxygen that makes up the atmosphere on earth.  The amount of oxygen the water can hold depends on the water's properties.  For example, increasing the temperature and the salinity of water will decrease the water's ability to hold oxygen.  Dissolved oxygen in aquaculture is one of the most critical parameters.  There are many products currently available for quickly checking DO, using test kits or probes.  Many biologists accept 6 ppm as the minimum concentration of DO needed to support fish life.
    Closely related to DO, is the need aquatic organisms in any system have for oxygen, which is known as the biological oxygen demand (BOD).  Aeration devices are used in aquaculturee due to the high BOD levels in most systems as a result of high density stocking of the cultured organism and the high use of oxygen by bacteria breaking down wastes in the system.  When using aeration devices to decrease the BOD (or to increase the DO), there are at least 5 criteria to be observed which will affect the ability of the water to hold oxygen.  These 5 are:

1.  Temperature - the higher the temperature, the less oxygen can be held
2.  Salinity - the higher the concentrations of dissolved salts, the less oxygen can be held in water.
3.  The amount of air coming in contact with water, so there must be an adequate amount of oxygen present.
4.  A large enough surface area of oxygen/water contact to allow exchange.  For example, one liter of air making 10 large bubbles has less bubble surface area than one liter making 10000 bubbles, so with the smaller bubbles more oxygen would dissolve into the water.
5.  The oxygen gradient between the water and air, os if there is little oxygen in the water, oxygen will enter faster than if the water is already saturated with oxygen.
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    Generally speaking, any fish's body temperature is about 0.5o C (1o F) higher than the temperature of the water it is in.  With this strong dependence on the temperature of the surrounding environment keeping the fish within the temperature range where they can survive, it is easy to see the importance of temperature in your system.  Fish are able to withstand some gradual changes in temperature, especially if this occurred in their natural habitat, but large changes in temperature, especially when transporting fish should be avoided.  In managing aquaculture systems, the sought after temperature is the optimal growth temperature.  This Temperature varies between species, but always represents the temperature at which aquatic species can focus energy on growth and not on staying water to stay alive.  In actual aquaculture systems, the temperature is usually kept somewhere below the optimal growth temperature due to other potential hazards associated with high heat.  Temperature can be continuously monitored with a temperature probe or a low tech thermometer.

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    These refer to molecules in the water that can be used as nutrients for plants and phytoplankton.  Two of the most important of these nutrients are nitrogen and phosphorus.  Nitrogen is used by organisms in many chemical reaction, and is found as a building block of amino acids in proteins.  It is most commonly found as nitrogen gas (N2) in the atmosphere.  However, the form of nitrogen excreted by animals is most commonly ammonia (NH3).  This is a toxic compound when its concentration in the water becomes elevated.  The nitrogen cycle shows how bacteria in the system can adequately  break down ammonia into the more usable and less toxic form of nitrate (NO3).  This cycle is also alluded to in the discussion of biofiltration on the startup page, which explains the removal of nitrogen from the system.  Ammonia, nitrite (NO2, an intermediate in the nitrogen cycle), and nitrate can be tested for using test kits.

An ammonia test kit

    Phosphates are less of a threat to aquaculture farmers since the largest problem associated with high phosphorus and limited nitrogen is cyanobacteria blooming.  This can cause the release of toxins and raise the BOD of the system, but these affects aren't a critical as high levels of ammonia.

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