We have moved again - now at St Kilda. You can contact us on 0411 105 365. We are in the process of setting up hydroponic and aquaculture displays.

Monday 0930 to 1730
Friday 0930 to 1730
Saturday 0930 to 1230

We are available for consultation and site visits at these times- other times by appointment by email (robin@soladome.com.au)

After 35 years at 44 Chapel Street it was time for a change to a web based operation and to offer guidance based on many years of practical experience.



Information & Education

Some technical terms, conversion tables, maths and concepts that are handy to know about. Also includes more detailed technical and practical advice on how to start your system.

Use the article index on the right to navigate between pages


{mospagebreak title=Common Terms}

Common Terms

DO (dissolved oxygen)

Measured either as an absolute value in milligrams of Oxygen per litre of water (mg/l) or as a relative value - the percentage of Oxygen dissolved in the water compared to the maximum amount of Oxygen that could be dissolved in the water at that temperature and pressure.

Typical values in a reasonable recirculating system would be 8 mg/l or 85 - 95% at 24 degrees Celcius

pH (the measure used for how acid or alkaline the water is) If you are feeding heavily expect the pH to keep dropping and you will need to add bicarbonate of soda to maintain the pH at the target level.

Temperature - usually measured in degrees Celcius.

Temperature is one of the critical water quality parameters and is often not given enough attention. Fish generally require a very narrow range of temperatures for optimum feeding and growth particularly in their early development stages.

EC ( Electro Conductivity ) measured in milliSiemens per Centimeter squared ( mS/cm2 ) or in microSiemens per Centimeter squared

as a rough guide 1 EC unit ( 1 mS/cm2) is equivalent to 640 PPM ( parts per million ) of dissolved salts

Salinity - expressed as either PPT ( parts per thousand ) or as a percentage salinity

Seawater is about 35,000 PPM ( 35 PPT ) or 3.5%

TAN - total ammonia nitrogen ( includes ionised and un-ionised ammonium ) the un-ionised part is very toxic to most aquatic animals and the percentage of un-ionised ammonium increases as temperature and pH increase.

NO2 - nitrite - toxic to aquatic animals at very low levels - parts per million

NO3 - nitrate - not so toxic but levels generally need to be below 150 PPM (parts per million)


{mospagebreak title=Measurements}


mg/l and PPM are roughly equivalent

HRT - hydraulic retention time - how quickly the water is recirculated in a system - generally every 60 minutes

SSA - specific surface area - the measurement of bio-filtration material - the surface area in metres squared of a cubic meter of the material - generally values greater than 260 m2/m3 are usefull

DNR - design nitrification rate - measured in gms/m2 - the number of grams of TAN consumed by a bio-filter per day - generally measured as grams of TAN consumed by m2 (square metres of surface area) of biofilter medium per day.

FCR - Food Conversion Rate - the rate at which food fed to the fish is converted to increased fish biomass - a rate of 1.5 : 1 means 1.5 kgs of food is converted by the fish to 1 kg of increased weight.

{mospagebreak title=Mathematics}


Lengths - measured in metres and millimeters - there are 1000 millimetres in a metre

Depths - measured in metres and decimal fractions - eg 1.25 metres deep

Areas - measured in square metres ( m2 )

Volumes - measured in cubic centimeters, litres and cubic metres. - there are 1000 cubic centimeters in a litre and 1000 litres in a cubic metre.

Weights - measured in grams and kilograms and tonnes - there are 1000 grams in a kilogram and 1000 kilograms in a tonne.

Percentages - the number of units measured divided by 100 of those units

1 percent written as 1% is the same as 1/100

15 percent written as 15 % is the same as 15/100


  • 10 is what you get when 1 is multiplied by 10 - it can also be called 10 to the power of 1
  • 100 is what you get when 1 is multiplied by 100 - it can also be called 10 to the power of 2
  • 1000 is what you get when 1 is multiplied by 1000 - it can also be called 10 to the power of 3
  • 0.100 is what you get when 1 is divided by 10 - it can also be called 10 to the power of minus 1
  • 0.010 is what you get when 1 is divided by 100 - it can also be called 10 to the power of minus 2
  • 0.001 is what you get when 1 is divided by 1000 - it can also be called 10 to the power of minus 3

Many results of testing water used in aquaculture use measurements like parts per million ( PPM ) or milligrams per litre ( mg/l )

A milligram is a thousandth of a gram ( 1/1000 gm ) so one milligram in one litre of water is 1 unit in 1/1000 x 1000 which is 1/1,000,000 - one part in one million parts

A molar solution of a substance is one molecule of the substance in one litre of water with the molecule being the sum of the atomic masses in grams. A molecule of salt is NaCl ( one sodium atom joined to one chloride atom ) So a molar solution of salt would be the atomic mass of sodium ( Na ) 23 plus atomic mass of chloride ( Cl ) 35.5 which equals 58.5 grams of salt dissolved in one litre of pure water.

A millimole solution would be a molar solution divided by 1000

{mospagebreak title=Water measurements}

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

{mospagebreak title=Water temperature, Hardness, Buffering} 

Each aquatic animal species has a preferred water temperature range. This preferred temperature range may vary at different stages of the life cycle. Animals need to be kept in their preferred temperature range for optimum growth. Animals kept in temperatures lower than their preferred range often stop feeding.

Organic processes are dependant upon temperature - eg increase the water temperature by 4 degrees Celcius and the bacteria activity can double.

Water Hardness

Rain water is considered to be soft as it lathers easily with soap and has low levels of Calcium and Magnesium salts.

Water hardness is measured in different units which vary from country to country. (See conversion table below ) Generally only the statutory units ( SI units ) are used in business and commerce. The SI units are measured in mol/l.

Alkaline Alkaline German ppm of British French earth earth degree of of degree degree ions ions hardness CaCO3 mmole/l meq/l d

  • 1 mmole/l of alkaline earth 1.00 2.00 5.6 100.00 7.02 10.00 ions
  • 1 meq/l of alkaline earth 0.50 1.00 2.80 50.00 3.51 5.00 ions
  • 1 German degree of hardness d 0.18 0.357 1.00 17.80 1.25 1.78
  • 1 ppm of CaO3 0.01 0.020 1.056 1.00 0.0702 0.10
  • 1 British degree 0.14 0.285 0.798 14.30 1.0 1.43
  • 1 French degree 0.10 0.200 0.560 10.00 0.702 1.0
  • Total hardness ( degrees of d ) Evaluation
    0 - 4 very soft
    4 - 8 soft
    8 - 18 medium hard
    18 - 30 hard
    30 + very hard
  • meq/l Milligram-equivalents per litre
  • mmole/l Millimols per litre

When using the German hardness units ( d ) note that no CaO is present in aqueous solution but that all hardness formers are calculated as CaO

(Source Merk )


Water that has high levels of Calcium Carbonate ( CaCO3 ) is considered to be well buffered as it resists changes in pH ( the level of acidity or alkalinity )

{mospagebreak title=Starting your system} 

Starting your system

First wash all the tank, pumps, pipes, hoses and fittings with a mild bleach solution ( 5 ml household bleach per one litre of water) to sterilise the system to remove any diseases or pathogens. Wear eye protection and plastic or rubber gloves to avoid skin and eye damage. When everything has been cleaned and dried then wash well with plain water.

Connect everything up and fill with the water you are going to rear the fish in, turn on the aerators, heaters and pumps and check that there are no leaks and the pH, temperature, salinity and oxygen levels are at the levels to suit the fish you are going to put in the system.

The biofilter now needs to be conditioned using a nitrifying bacteria culture or you can plan to let your biofilter start working naturally and develop with your fish.

The fish arrive usually in a plastic bag inside a foam box. Take the plastic bag of fish and float it in the tank, carefully open the top of the bag and slowly, over a period of about 30 minutes add water from your system into the bag so that the fish become acclimatised to the new environment.

Slowly lower the bag into your system and let the fish swim out of the bag into the system. The fish will generally be pretty stressed at this stage so expect to see them rush for the bottom of the tank and then stay still for a while.

After a short time they will start moving and exploring but it may be some hours before they are ready to eat any food.

The first few days and weeks are critical and you will need to be doing a lot of checking of the water quality and biofilter to make sure that the system settles in.

As soon as the fish enter the system and you begin to feed then there are waste products produced in the form of fish wastes and uneaten food. A group of bacteria called heterotrophic bacteria will attack these wastes and convert them to ammonia type products.

Small quantities of ammonia products ( 1 part ammonia per million parts of water) in the water are lethal to aquatic organisms.

Fortunately your biofilter will handle this problem so long as there is enough surface area, air and moisture for the biofilter bacteria to live and thrive.

The biofilter bacteria are special and called autotrophic bacteria - they consume ammonia products dissolved in the water and convert them first to nitrite ( also very toxic to aquatic organisms - few parts nitrite per million parts of water ) and then to nitrate which is much safer ( can have up to 400 parts of nitrate per million parts of water for some species of aquatic animals.)

Now some very general rules of thumb

  • Move the water through the filtration and biofiltration system about once an hour.
  • Keep the pH within 0.2 points
  • Keep the temperature wiithin 1 degree Celsius of your selected temperature
  • Keep the DO ( dissolved oxygen levels ) above 70% saturation
  • Keep the salinity within 0.1 mS/cm2 (about plus or minus 50 parts per million of your selected level)
  • Have the biofilter volume equivalent to 20% of the tank volume
  • Keep the daily feeding rate steady or only alter it slowly
  • Keep the light levels low
  • Keep the feeding schedule constant day by day
  • Exchange 5 - 10 % of the system water per day

Here is a typical starting sequence for some saline fingerlings

The fish arrived in full seawater but the eventual environment was to be saline - about 1/4 seawater or EC 12 mS/cm2 ( 7.5 parts salt per thousand parts of water)

The system was filled with seawater and warmed to 24 degrees Celsius, Aerators switched on, pump switched on and biofilter connected.

Fish in their plastic travelling bag were floated in system tank, bag top opened and during next half hour system water gradually added to bag. Fish then floated out of bag into system. Fish immediately went to bottom of tank and stayed very still. This is a typical shock reaction. After a couple of hours the fish start to explore their new habitat and may respond to a light feed.

For maximum growth the fish need to be fed regularly with a high protein diet. If you have time then feed the fish several times a day until they stop feeding. Keep accurate records so that you can calculate the rate of growth and the food conversion ratio (FCR) - how many grams of feed required for the fish to increase their weight by one gram. As a guide very small fish may consume up to 15% of their bodyweight per day in food. larger fish (150 - 250 gms) may be consuming 5 - 8 % of their bodyweight per day. Larger fish 600 gms upwards may only consume 1 - 2 % of their bodyweight per day. Good fish husbandry is all about knowing your fish and responding quickly to their needs.

There are quick and expensive and slow and cheap ways to get the bacteria in your biofilter growing.

The quick and expensive method is to purchase a good live nitrifying bacteria culture and bacteria food and innoculate your biofilter so that within a couple of days the nitrifying bacteria numbers have grown to handle the ammonia formed in your system.

The slow and cheap method means that you have to let the bacteria populations build up slowly to handle the ammonia load in your system.

BUT both methods require you to take regular (at least daily) readings of your water quality to monitor the development of the bacteria in your biofilter.

What you are looking for is a gradual rise in the TAN (total ammonia nitrogen) with a nil or very low value for nitrite and nitrate. As the TAN level rises you will have to carry out water changes to keep the TAN level below the lethal level for your fish species.

After a couple of weeks (slow method) the TAN level peaks as the nitrite level starts to climb. Again you have to keep water exchanging to keep the nitrite level below the lethal level for your fish. After another couple of weeks the TAN has fallen nicely and the nitrite peaks with the nitrate starting to rise. Again water exchanging will be required to keep the nitrite levels down. Another week or so and the TAN is low, the nitrite is low and the nitrate is rising slowly. At this stage your biofilter is working well.

BUT if you change the number of fish or the amount of food fed or change the water quality parameters then do so slowly - allow the biofilter bacteria time to get used to the new food levels.

The quick method speeds up the process rapidly and allows you to stock your system with more fish quicker. You still have to do the daily water quality monitoring to make sure that TAN, nitrite and nitrate levels go through the same rises, peaks and falls as with the slow system.

Our experience has been that the Nitrosomonas bacteria (the ones that convert ammonia to nitrite) get up and going relatively quickly, however the nitrobacter bacteria (the ones that convert nitrite to nitrate) are much slower to get established so one ends up with a long period where the nitrite levels keep rising and you just have to keep daily water exchanges to keep the nitrite levels within limits for your fish

Regular water quality checks are very important and you should check the following on a daily basis

  • pH, temperature, salinity, dissolved oxygen, amount of food and feeding times, water taken out, water added, bicarbonate added.
  • When getting the system started you also need to check DO, TAN, NO2, NO3, CO2, Alkalinity
  • Changing salinity
  • Mortalities

Mortalities are an unfortunate fact of life when your experience and equipment are untried. Keep an accurate record of your mortalities, numbers, size, weight, condition and if possible reason for death. Try not to make the same mistake twice. In many case mortalities can be avoided by thinking and planning ahead.