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Water Chemistry: pH, GH and KH What are they all? -   Scott Thomson 

Copyright © 2003 World Chelonian Trust. All rights reserved


Related articles:

Understanding Biological Filtration - Jody Karlin 

Introduction :

The water chemistry requirements for turtles are often underestimated. At the same time it needs to be acknowledged that they are not fish and hence are not susceptible to going belly up from a rapid pH change.

Some species do benefit from a more exacting regime, and it can be said that there are health benefits, even if indirect, from careful monitoring and adjustment of these water quality parameters.

First of all I will define these terms as I think we all probably have some knowledge of what pH means but some detailed knowledge is required.

Per Hydrogen (pH)

The pH of the water is a measure of the balance between the Hydrogen (H+) and Hydroxide (OH) ions in the water. I think most of us know that low pH is acidic and high pH is alkaline or basic. Hence a pH of 5 is slightly acidic water, a pH of 7 is neutral and a pH of 8 is alkaline water.

For the budding chemists the equation is as follows:


pH = -log10[H3O+]


All right enough of the chemistry but there is an important point here and any mathematicians will see it. The pH scale is a logarithmic scale. In other words a pH of 6.0 is 10 times more acidic than a pH of 7.0 and a pH of 5.0 is 100 times more acidic than a pH of 7.0. As a keeper you need to be aware that it is not a difference of 1 when you go from 6.0 to 7.0.


Ways to lower pH

Filtering water over peat
Add bogwood to the tank
Inject carbon dioxide CO2
Use a commercial acid buffer
Water changes with softened water or RO (Reverse Osmosis) water

Ways to raise the pH

Aerate the water, driving off the carbon dioxide (CO2)
Filter over coral or limestone
Add rocks containing limestone to the tank or use a coral sand substrate
Use a commercial alkaline buffer



Carbonate Hardness (KH)

This is an area where many people get confused. One of the reasons the term alkaline is avoided a bit and the term Basic is used for the pH scale is because this reading is measuring the alkalinity of the water. It is not the same as alkaline.


The alkalinity is a measurement of the waters buffering ability, or its ability to absorb and neutralise acid. Clearly the more alkalinity or the higher the Carbonate Hardness of the water the less likely you will incur pH swings in the water. It is therefore important to get this figure reasonably high to stabilise the water.


Exactly how high you want your dKH (degrees of KH) will depend on what pH you choose to use. To people keeping fish from the African Lakes this is the life and death of their fish. Those fish live in very stable high pH conditions, they do not like change and the conditions can be difficult to imitate. Fortunately turtles are not quite so sensitive. However, I do think that basic fish keeping can teach the aquatic turtle keeper a thing or two about water.


Ways to increase kH

Adding sodium bicarbonate (baking soda). One teaspoon of baking soda added to 50 liters of water can raise the kH of the water by approx 4 deg dH without a major affect on pH.
Adding an air stone to increase surface turbulence driving off carbon dioxide (CO2)
Adding commercially available products to increase buffering capacity


Ways to lower kH

Injecting carbon dioxide (CO2)
Use reverse osmosis (RO) water. You can mix tap water with reverse osmosis water to achieve the desired kH.
Adding commercially available products to decrease the buffering capacity.



Do not use distilled water as it has no dissolved salts and hence no buffering ability. Add a small amount of acid (eg uric acid) and it will shift the pH very rapidly. It is also highly osmotic and will react with the turtles renal system.

General Hardness (GH)

This is essentially a measurement of Magnesium and Calcium ions in the water. Again it is measured in the German degrees of hardness scale or parts per million. This is what is generally meant by soft and hard water which are terms people should be familiar with. The table below shows comparisons between parts per million, the dH scale and the generalised concepts of soft and hard water.



Ways to increase gH

Adding limestone to the aquarium (this will also increase kH which in turn will increase pH)
Adding calcium carbonate will raise gH and kH


Ways to reduce gH

Adding peat moss to your filter
Use commercially available water softening pillows or a water softener (this removes calcium and magnesium ions and replaces them with sodium ions. Many people feels that this is an unacceptable method of softening water as many fish that prefer soft water donít like sodium either.
Mixing tap water with reverse osmosis (RO) water.



General Hardness Table

  0 to 4 dH 0 to 70 ppm Very Soft  
  4 to 8 dH 70 to 140 ppm Soft  
  8 to 12 dH 140 to 210 ppm Medium Hard  
  12 to 18 dH 210 to 320 ppm Fairly Hard  
  18 to 30 dH 320 to 530 ppm Hard  


So what does this mean to the Turtle.

Here is the complicated part, should you worry about the pH for a reptile. I would argue yes you should and for a number of reasons.

    1. A stable pH environment is indicative of good water quality, a working filtration system and can put you at ease that your turtles are in a good healthy environment. A healthy environment generally means healthy turtles. If the pH suddenly swings it tells you something is wrong, before the turtle is sick.

    2. Aquatic turtles react osmotically with their environment via the cloaca. By making this reaction in the turtles favour you increase the efficiency of the water turnover in the turtle. I am not saying they cannot live in variable conditions but if you have them in captivity why not make them have a good healthy life.

    3. Many disease agents that affect turtles do best around neutral water. As we are keeping turtles and not fungal spores why not have the water conditions set to minimise the risks of infections.

Therefore I tend to recommend high pH conditions for turtles. Certainly Neutral and above anyway. I do not think acidic conditions are particularly good for turtles and they also seem to do better in moderately hard water, rather than soft.

In saying all this I will make a note for one species: the pig nose turtle (Carettochelys insculpta) does seem to be a hardwater, high pH specialist. In the wild they are restricted to limestone based rivers which have pH readings of 8.0 to 8.3. This species seems to be very susceptible to fungal infections and hence I would recommend the high pH for this species. A similar argument can be made for members of the Chelodina.

One point must be made and that is that if you maintain the water above 7.6 pH then you need to have a good biological filter. This is because Ammonia, which is toxic in itself, is converted to Ammonium at 7.6 and Ammonium is extremely toxic.

Testing the Water.

This is actually relatively simple. The commercially available aquarium pH and Hardness test kits are certainly accurate enough. Follow the directions on whichever ones you buy and do the tests once per week to monitor the aquarium. Remember that when doing water changes you need to prepare the water so that itís the same as what you removed. So if you add salt you need to add more. If you are maintaining a high pH and your local water is soft and acidic you must correct this first.

For the maintenance of high pH freshwater you should obtain a Marine pH test kit as the freshwater ones usually only go up to 7.8.



Put simply we all know we are not keeping fish which have a very intimate relationship with water condition. But it is still important to have good quality, clean, stable water for your turtles. I do recommend high pH and hardness for turtles, as I have said this is not because they cannot live in other conditions but because they do well in captivity in this regime.

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