pH... and how to change it
pH is commonly referred to as a measure of the acidity or alkalinity of a solution (although a more
scientific description would be that pH is a measure of the concentration of Hydrogen atoms, H+).
The pH scale runs from 0 (acidic) to 14 (alkaline), with neutral pH 7 in the middle. The pH scale is
logarithmic, which means, for example, that pH 5 is ten times more acidic than pH 6.
The pH range of interest to the tropical fishkeeper is between pH 5 and pH 9, with the vast majority
of fish requiring a pH between pH 6 and 8. The pH value of the water has an important influence on the
way a fishes body functions.
It is useful to know the pH of your tapwater (or other source), to determine its suitability for the fish you wish to keep.
Measure the pH after the water has been left in a bucket overnight, as the pH may change due to gases
dissolving into, or being released from, the water.
Changing aquarium pH
Before attempting to change the pH of aquarium water, you should ask yourself if it is really necessary
to do so. The pH range quoted for a given species may be based on its native waters. Although it might
be desirable to mimic these conditions to some extent, the fish may be quite capable of thriving at a
slightly different pH. It is also worth considering that many fish acquired in the hobby may have been
aquarium bred for many generations and already become accustomed to water conditions quite different
to their natural habitat. Providing a stable pH is usually more important than the exact value, as long
as extremes are avoided.
There are of course some fish which do require specific conditions to thrive. You may also want to alter
water chemistry to improve success with breeding, or to improve the growth of demanding plants.
Increasing pH is usually easier than lowering it, and will usually involve raising hardness at the same
time, in order to keep the pH stable. It can be achieved in the following ways:
Lowering pH can be more difficult, particularly in hard water which has a good buffering capacity.
The following methods are sometimes employed.
- The use of decor containing buffering salts, such as limestone rock.
- The use of crushed coral in the filter.
- Commercial buffers and "pH-up" products.
- Filtration through peat, this will be more effective in water with a lower KH.
- Commercial "pH-down" products. Again, these will not work effectively where there is a strong
buffering capacity. Some will also introduce phosphate to the aquarium which will encourage undesirable algae
Attempting to lower the pH of well-buffered water with commercial chemicals or acid solutions
is likely to result in a losing battle, as the buffer system causes the pH to restabilise at
its original value. The resultant pH swings are likely to be harmful to fish. The solution is
reduce the buffering capacity or carbonate hardness (KH) first.
Apart from the effect of pH itself, there are important effects on the toxicity of ammonia and nitrite
with changing pH. Therefore you should be particularly wary of attempting to change pH when either of
these waste products is detectable - in particular, during the cycle. It is
safer to let the cycle finish before attempting to adjust pH - it may settle at a different value once
the cycle is complete in any case.