Aquarium Water Chemistry 

Aquarium Water Chemistry


PH Defines how acidic or alkaline the water is. It equates to the amount of hydrogen (H+) and hydroxide (OH-) ions are dissolved in a solution.  The more hydrogen ions there are, the more acidic the water is and the lower the pH is. A solution that has equal concentrations of hydroxide and hydrogen is termed neutral with a pH value of 7.  A higher concentration of hydroxide ions would return a value above 7 or alkaline Lake Malawi has a pH of around 8.3 (it ranges between 7.8  -8.5), it is recommended that the pH of the aquarium water be maintained between 7.0 and 8.5, it is wise to to attempt to match the aquarium water with that of the lake.

Water that is poorly buffered (low KH) will be subject to higher PH fluctuations than well-buffered water.  As a general rule, hard water is usually alkaline (above 7) and well buffered, whereas soft water (below 7) is usually slightly acidic and poorly buffered.  KH also has an affect on the pH.

Fluctuations in pH can be stressful and damaging to fish health also Nitrifying bacteria, essential in the conversion of ammonia to nitrate also have a pH range preference, which is between 7.5 and 8.6.  Variations in pH will also have an effect on some disease treatments.


Carbonate hardness or temporary hardness. Measures the buffering capacity or the ability to absorb and neutralize added acid without major changes to PH, the nitrogen cycle in our tanks produces nitric acid (nitrate).  If we don’t have buffering (KH), the PH will drop over time.


General hardness (GH) refers to the dissolved concentration primarily of magnesium and calcium ions.  When fish are said to prefer ``soft'' or ``hard'' water, it is HH, not KH that is being referred to.  GH will not directly affect PH although "hard" water is generally alkaline due to some interaction of GH and KH.

Incorrect GH will affect the transfer of nutrients and waste products through cell membranes and can affect egg fertility, proper functioning of internal organs such as kidneys and growth.  Within reason, most fish and plants can successfully adapt to local GH conditions, although breeding may be impaired.

Making Changes to the Water Chemistry

Ways to raise the PH
- Place coral gravel or limestone chips in the filter as media. (these will continually "leech" out minerals to keep the water on
the alkaline side.* This will only help and will not be enough to raise the PH alone.)
- Use a commercial alkaline buffer
- Use common bicarbonate of soda (recommended)

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 dH without a major affect on pH.(recommended)
- Adding commercial products to increase buffering capacity

Ways to increase gH
- Adding Epsom Salts (Magnesium Sulphate) will raise GH (recommended)

- Adding Malawi Salts (Mineral to replicate the Lake) will raise GH (recommended)

Home Made "Buffer Recipe"

To raise KH and PH, add Bi-Carbonate of Soda. A recommendation is 1 teaspoon per 5 (US) gallons of water (dissolve in a jug of  warm aquarium water). This can be syphoned into the tank with a piece of airline and preferably in the strongest flow by the filter inlet. It is important you monitor the PH throughout this process which will mean quite a few tests. This is where a PH monitor comes in handy so if you've got one this process is easy.
Once you reach the 0.3 increase stop, and repeat similarly on each consecutive day until your PH reaches the required level.

Repeat this procedure for Epsom Salt, starting out with ½ teaspoon . Try to achieve a GH between 15 and 25 dgh.

The API Liquid Test Kits are Highly recommended for testing both KH and GH Both can be bought together for around £6.00

The important thing to remember is consistency should be maintained when using any of the methods used, each time water is changed additional Epsom Salts/BiCarb should be added to buffer the new water back up.