Water Chemistry

Firstly, you must know that fish are highly dependent on the water in which they live.

Any changes in their water will have a direct impact on their health and well-being.

These changes are often invisible to the naked eye, so it important to do regular tests.

It is best to familiarise yourself with fish care before you start, as fish heath will determine your overall success with aquaponics. Please remember these are living creatures, if testing water sounds like too much hard work, aquaponics might not be for you.

Note: It is essential to test water before any fish are introduced you will need to buy a freshwater aquarium testing kit and control correct levels of each for your specific type of fish.



YOU WILL NEED TO TESTwater chemistry

Water Temperature (using a thermometer)


Toxic waste Ammonia (NH3), Nitrite (NO2-) Nitrate (NO3-)


PH (alkalinity/acidity)– these levels can be attributed to by certain rocks/grow medium (e.g rock wool or limestone) you place in your system





Different area’s have different water. It is good to know what area you are in, as this may affect your systems performance. General Hardness is the total of all dissolved minerals in your water, principally calcium and magnesium salts, so testing your system for this will help you understand more about your plant’s requirements.

For Hard water areas (increased acidification, rich in calcium) either source softer water for water changes, boil water before use.

For soft water areas (fewer dissolved salts, with lower pH): you may require a pH buffer (e.g aquarium salt)

Carbonate hardness is one of the most important factors for plant growth, and should be kept at medium-soft levels.

Never add tap water directly to your system.




Your fish live in a closed system, which means that the waste matter they excrete can build up relatively quickly to dangerous levels, unless managed carefully.

By testing the water any changes can quickly be identified and remedied before they lead to fish health (and therefore plant health) problems. Do this until your system consistently provides the readings you require. After this, you only need to test every week or immediately if there are any visible signs of change in plant health (e.g. yellowing) or fish behaviour  (e.g. frequent gill movement) occur.




The most significant waste, as it is very dangerous to fish. Any level of ammonia suggests something is wrong.  Unfortunately, ammonia is a natural by-product of many things in your tank decomposing; such as dead plants, uneaten food, animal tissue and fish faeces. The exact level of toxicity (total NH3 & NH4 level measured in milligrams per lire) – depends on the water’s pH and temperature. Ammonia is most dangerous with high pH and high temperature. At low levels, it can cause illness and at high levels can kill them.


Ammonia is broken down by naturally occurring bacteria to produce Nitrite – fish prefer no nitrites. (0 milligrams per litre) above this, the water conditions become dangerous to the fish.

0 mg/l = ideal conditions – well done!

0.1 mg/l = acceptable conditions, but need rectifying

0.25 – 0.5 mg/l = Poor conditions. Excessive  level of nitrite, take immediate action.

2-4 mg/l = water conditions hazardous to fish health – take immediate action to prevent death.

Two main reasons for underlying ammonia/nitrite problems:

Overfeeding  Too much excess organic material in the biological system

Bacteria die due to unsuitable conditions:

  • low oxygen, a pH below 6.5
  • Chemicals such as household cleaner
  •  Fish medicines.


Is one of the main causes of algae problems in aquariums. It is much less toxic then ammonia and nitrite, however high levels do cause stress which can to lead to disease. Nitrates build up slowly, caused by successive breakdown of waste material.

0-10mg/l = ideal conditions – nice one!

10-50mg/l = acceptable conditions, but may stress fingerlings/small fry

50-75mg/l = poor conditions. Will stress most fish, and may cause algae problems

75 – 100 mg/l = water conditions hazardous. Take immediate action to reduce these levels



Water is made up of positively charged hydrogen ions (H+) and negatively charged hydroxyl ions (OH-) The pH measures the ratio of the two in a body of water – pH tests measure the amount of  hydrogen in the water.

The scale of 0-14 is logarithmic (e.g. pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6.)  this measures the acidity/alkalinity of the water:


0 – 6.9 = Acidic  (contains more hydrogen ions)

7.0 = Neutral (equal ratio of hydrogen and hydroxyl ions)

7.1 – 14 = Alkaline (contains more hydroxyl ions)


The pH of aquarium water is constantly changing due to several factors: photosynthesis by the plants and algae, waste produced by the fish and evaporation.  If you do experience undesirable pH, it is important to take action, and work out what has caused the problem.

 Most plants prefer pH 6.5 but some will tolerate more alkaline conditions. Some plants  will get nutrient lock-out (stop taking up certain nutrients) around pH 8.0 it is important to find a balance for them and your fish so both grow happily.

Fresh water fish require a pH of between 6.5 and 8.5. Outside of these levels, fish will be prone to stress, as well as physical fin/tissue  damage, which in turn, is likely to leave them susceptible to disease.  At extreme levels below 5 or above 10 fish will be likely to die,  Obviously this could then affect your produce too– all of which is a less than desirable outcome!

Lowering pH (making it more acidic)

pH is closely linked to C02 levels, because carbon dioxide produces carbonic acid, which lowers pH, so measuring C02 levels is a good idea. If a system is young, and has slightly high alkalinity, don’t worry too much. It is worth identifying what you think could be causing it, such as rocks or shells, but it will most likely settle down itself. If you feel it urgently needs lowering try:

Removing gravel, shells or ornaments

pH down

! Hydrochloric acid (highly corrosive!)

Iron sulphate

pH can also drop if C02 levels increase

Raising pH (making it more alkaline)

pH up

egg shells / sea shells*/ snail shells

Potassium carbonate

Potassium hydroxide (KOH / pot ash)

*sea shells, or stones should be gently boiled as they may contain harmfull bacteria

Common causes of acidity: may be due to overstocking,  stones/medium added to aquarium leeching or decomposing plant matter.

Common causes of alkalinity: stones/ornaments/medium added to aquarium, excessive plant growth.


Tip: It is good to consider which plants will thrive  in your fish’s pH range and water temperature.






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