Water Measurements

Pure water is odourless, colourless, has no turbidity, contains no dissolved salts or bacteria and is made up of many molecules of hydrogen and oxygen combined in the ratio of two hydrogen atoms joined to one oxygen molecule Written as H2O or HOH

In pure water one molecule in 10 million " ionizes " or separates into a hydrogen ion and a hydroxyl ion written as H+ and OH- . The + signifies that the hydrogen ion has a positive charge ( is short of one electron ) and the - signifies that the hydroxyl ion has a negative charge ( has an extra electron ).

One molecule in 10 million can be written as 1 in 10,000,000 or 1/10,000,000

A number 1 followed by seven zeros can also be called 10 to power of seven

The amount of ionization in pure water is 1 in 10,000,000 so the fraction is 1/10,000,000 which is also called 10 to power of -7

See separate notes on pH

Pure water will not conduct electricity

Most water used for aquaculture has some impurities dissolved in it.

For example if the water contains a little salt then the dissolved salt molecule will also ionize. Salt in crystalline form is NaCl ( one sodium atom combined with one chloride atom ) In water the NaCl ionizes to Na+ ion and a Cl- ion.

If two metal rods are placed in this solution and one rod has a positive charge and the other a negative charge then a current will flow between the two rods. The Na+ ion will be attracted to the negative rod and the Cl- ion will be attracted to the positive rod. The positive charged rod is called an anode and the negative charged rod is called a cathode. The + charged ions are called cations ( short for cathode ions ) and the - charged ions are called anions ( short for anode ions ).

The more chemicals that are dissolved in the water the more ions there are - equal numbers of cations and anions. The more ions there are in solution the easier it is to pass an electric current through the liquid.

Using two metal rods it is possible to see how fast a current of electricity can move between the two probes - the more ions in solution the faster the current will flow.

This increase in electric current speed with increase in number of ions is used to measure the " saltiness " or salinity of the water.

See separate notes on salinity and Electro Conductivity ( E.C.)

As well as salts dissolved in water gases can also be dissolved in water. The common gases measured in aquaculture water are oxygen and carbon dioxide.

Oxygen - this gas is composed of two oxygen atoms joined together written as O2 . In fresh air oxygen makes up 21 % of the total. So in 1000 litres of air there will be 210 litres of oxygen ( Oxygen weighs 0.666 gms per litre at 24 degrees Celcius and standard pressure )

Water can only hold a very small amount of oxygen so that in 1000 litres of water at 24 degrees Celcius there may be only 10 gms of oxygen equivalent to 2.5 ccs ( cubic centimeters )

The higher the water temperature the less oxygen gas that can be dissolved in that water.

In aquaculture the amount of dissolved oxygen gas in the water is measured either as an absolute value - eg 10 milligrams of oxygen per litre of water ( mg/l ) or as a relative percentage value - the amount of oxygen gas dissolved in the water compared to the maximum amount that the water could hold at that temperature and air pressure. The latter measurement is expressed as a percentage eg 89% oxygen saturation.

See separate notes on Oxygen and water.

Fish and aquatic animals in water produce waste products and the uneaten food particles also break down. Some of the waste products are visible as scum or particles suspended in the water or lying on the bottom. Other waste products are soluble in the water. All the waste products need to be removed or converted to relatively harmless by-products otherwise the waste products can quickly build up to toxic levels.

In a recirculating aquatic system the solids need to be removed as quickly as possible before they are churned up into smaller particles that can cloud the water and become harder and more expensive to remove. The suspended settleable solids need to be removed usualy by a screen filter mechanism or swirl separator. The dissolved organic waste products are next removed usually by using a protein skimmer ( bubbling air through the water to "foam" out the dissolved waste products ). The dissolved ammonia in the water from fish urine, faeces and uneaten food is passed through an active biological filter to convert the ammonia to nitrite ( NO2 ) and then nitrate ( NO3 ). The water needs to be oxygenated before returning to the system. Some heavily fed systems will need CO2 degassing systems to remove the build up of dissolved CO2 in the culture water.

Some systems use Ultraviolet and Ozone to treat the recirculating water to kill any undesirable pathogens or bacteria in the system. Ozone needs to be used with great care as levels above 350 millivolts can be harmful to fish

Fish need oxygen in the water to " breath " and to help food digestion. European experience suggests that the oxygen level in the outlet to the tanks needs to be a minimum of 70% saturation.

See separate notes on testing for Ammonium and Nitrite