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The pH of water is a measure of how acid or alkaline it is and is measured on a scale of 1 to 14.

Normally, the optimum pH for a community aquarium is pH7.0


Ranges from highly acidic-battery acid at pH 1.0, to highly alkaline-lye at pH 13.

Distilled water, at pH 7.0 is neutral, neither acidic nor alkaline


Fish in nature adapt to a wide range of pH values, but water that is too acid or too alkaline for the species, can severely stress fish and even cause death. Our experience has shown this to be a major cause of fish losses in aquariums and pet shops. Symptoms of fish subjected to pH extremes are similar to many fish diseases, and therefore a pH check is a must, before any medication of fish is commenced. Extreme pH symptoms include; clamped fins, heavy sliming, respiratory difficulties, frayed fins, listlessness, lack of appetite, and even death. Fish subjected to a gradual change in pH over many weeks can adapt, even though it may fall well outside their preferred range. Unfortunately, new fish introduced into this "unsuitable" water may die quickly. This phenomena explains many fish deaths, where the customer is sure his water is OK, because the old fish are alive, but the new ones die.


The best way to measure pH is with use of a simple pH test kit, using a narrow range indicator solution and a colour chart. pH indicators may deteriorate and become inaccurate due to incorrect storage or chemical contamination. Indicator liquids will change from their original colour if they are suspect.


Water from different areas varies considerably in its buffering capacity, and its pH. The normal pH range for natural water is usually between 4-9. The buffering capacity of water is a function of its carbonate hardness. The pH of water, low in carbonate hardness, is much more likely to fluctuate to an unacceptable level. For example, Melbourne tap water is very soft and has a low carbonate hardness and therefore has a low buffering capacity. This type of water, once in a stocked aquarium, progressively becomes more acid. This acidity is mostly caused by the action of the biological filter, and build up of carbon dioxide (carbonic acid) as the fish breathe.

Conversely, neutral tap water may become too alkaline for most fresh water species, by reacting with calcium bearing substances if present in the aquarium. The following items should not be used for fresh water tank decoration; sea sand, shell grit, sandstone, coral, shells, marble, and cement. Newly made cement ponds can be deadly for goldfish - pH readings of 10 have been recorded. Consult a good book on goldfish and ponds for more information on how to treat new cement ponds.


Instead of making constant adjustments to aquarium water, it is best to analyse the problem, find the cause and take a long-term approach to fixing it, as recommended in the next chapters. Any corrections made to the pH of an aquarium must be gradual. It is recommended that changes of no more than 0.5 units be made per day. (e.g. pH 6.0 to 6.5).

Carbon dioxide is produced when alkaline water is neutralised with an acid, this is another good reason to make the changes gradually, and keep the aeration going.


a)         If caused by the use of incorrect sand or rocks etc. (as previously described) simply remove offending material, do             partial water change, and buffer the tank back gradually using acid buffer.

b)         If tap water is too alkaline, it should be neutralised before it is used in the aquarium. A large plastic or fibreglass storage container should be installed, and used for adding acid buffer to the tap water. After adding acid, strongly aerate the container to drive off the resulting carbon dioxide.

Store neutralised water for several hours, and preferably up to 24 hours, before use. Always test pH again before adding to the aquarium.

The pH of tap water can suddenly jump to a lethal level, after work has been carried out on concrete or cement lined water pipes. Hence the importance of checking pH of your tap water before use.


Good quality German peat moss (available from garden supply shops) is useful as a natural method of acidifying water. By placing peat moss in any type of aquarium filter, water is softened slightly. The pH drops and water takes on a light brown colour, as it is stained by the tannins and humic acids.

This water is particularly good for breeding fish that require soft acid water. Do not leave peat moss in filters for more than a few weeks, because as the peat starts decomposing, it will release all the chemicals it has absorbed while being used as a filter.


a)         Make sure tank is not excessively dirty and that substrate filters are kept clean.

b)         Give tank a 1/3 water change (this is done to remove dissolved waste - it will not correct pH on its own unless the tap             water is alkaline).

c)         Add a neutraliser block, or approximately 1/4 cup of shell grit per bucket of sand used in the tank; these materials             gradually dissolve as water turns acid, automatically neutralising the water. It therefore becomes essential to do             weekly 1/3 water changes to prevent the accumulation of dissolved calcium, which increases general hardness.

d)         If water is highly acid and the fish are stressed, give tank a 1/3 water change and gradually add buffer or KH-up (remember no sudden change to pH).


The pH of an aquarium can be "locked in" (held steady) by the addition of KH-up (carbonate hardness tablets) and MpH lower". These two products produce an accurate "buffer" that holds pH stable for many weeks (see notes on carbonate hardness for more details).