The importance of pH
As koi keepers we are more water keepers than biologists,
and as water consists of H²O (as well as everything else dissolved within it)
we owe it to our koi to understand a little chemistry. This will then increase
our success and fulfilment, ultimately benefiting our koi.
Water is an amazing substance that makes life on earth
possible. Besides making a pond ‘wet’ and providing something for our koi to
swim in, water enables all the necessary chemical reactions to take place that
help maintain a pond’s health and balance.
Water is the world’s best solvent. You can tell where water
has been by analysing what it has dissolved in it. For this reason, the water
in an upland, mountain stream is relatively low in dissolved minerals, whereas
the opposite is true for a lowland river or lake. A koi pond is best modelled
on the chemistry and water quality found in a lowland lake as this is the
natural habitat of the wild-type ancestors of our koi.
Water and ions
A molecule of pure water (H²O) consists of two hydrogen
atoms joined to one oxygen atom. In a pond, the water molecules separate (or
dissociate) into ions – positively charged hydrogen ions (H+) and negatively
charged hydroxyl ions (OHˍ). These ions are constantly forming and reforming and
will do so with any ion of an equal and opposite charge. It is this
characteristic of water that makes it such an excellent solvent. For example, common salt – sodium
chloride(NaCI) will dissolve readily in water forming sodium ions (Na⁺) and
chloride ions (CI). This is true for many other elements and compounds, meaning
that each aquatic environment in the world, will have its own unique chemistry.
pH
pH is a measure of the acidity or alkalinity of a substance
and is an important measure for pond water. It is measured on a scale of 0 to
14, where 7 is neutral, below 7 is acidic and above 7 is alkaline.
Pure water
is neutral (neither acid or alkaline) and is the standard against which acidity
and alkalinity is measured. pH actually measures the quantity of free hydrogen
ions (the ‘H’ in pH) and recognises that free hydrogen ions make a substance
acidic. More hydrogen ions relative to hydroxyl ions will make the pond acidic,
more hydroxyl ions relative to hydrogen ions will make the pond more alkaline –
so pH is a matter of balance.
Taking lowland lakes as our pond’s model for
water quality, our pond’s pH must be alkaline – being stable between 7.0 and
8.5.
why koi prefer
slightly alkaline water?
As descendants of carp, our koi are a lowland, still water fish.
By the time fresh, clean and relatively pure mountain water has reached their
natural habitats, it would have picked up an array of salts and minerals,
making the lowland water alkaline and hard.
As their physiology has become
adapted to this water chemistry, a complete filter system must be provided with
a similar pH and water quality in ponds or they will become stressed.
The
target pH is between 7.0 and 8.5.
Processes in a pond
that can affect its pH
As pond keepers, we should aim for two things when managing
our pond chemistry.
·
A water quality that is within our koi’s natural
limits (i.e. what their physiology is adapted to and able to tolerate)
·
Having achieved a suitable water quality, that
it remains stable and within our fishes’ natural limits to avoid stress and
disease.
This especially applies to pH. There is a constant
tug-of-war occurring in koi pond where various natural factors conspire to
alter the pH with a tendency to cause fish problems. Essentially there are 3
problems that you may need to address in your pond.
·
The pH is consistently too low.
·
The pH is consistently too high
·
The pH is very unstable and prone to wild
fluctuations.
What if pH is less
than 7.0 and becomes acidic?
In effect, speaking chemically, this means that there is an
abundance of free H⁺ ions. A number of natural biological processes will put
pressure on the pH of a koi pond to become acidic.
a.
Biological filtration. When koi release ammonia
(NH³) they are releasing nitrogen and hydrogen ions. When that ammonia is
broken down by bacteria into nitrite (NO²) the three free hydrogen ions are
released into the water, causing a drop in pH. As ammonia excretion and
biofiltration proceed unabated in every pond, there is a relentless downward
pressure on pH in every pond. If the pH is allowed to drop below 7, then the
addition of further free hydrogen ions accelerates the drop in pH, leading to a
pH crash, causing your fish and other aquatic life real stress.
b.
Respiration. Plants (including blanketweed),
fish and bacteria respire constantly taking in oxygen and releasing carbon
dioxide. This process can also cause the pH to drop as the carbon dioxide
combines with water to form carbonic
acid. Excessive plant growth can cause the pH to drop to extremes at night,
rising again out of the danger zone in the day as plants utilise the carbon
dioxide in photosynthesis.
A drop in pH in a pond to below 7 will lead
to dramatic changes in koi health, particularly if they are long term.
Colours
in koi are likely to fade through the deposition of excess mucus while in
extreme cases, koi may even be seen to gasp at the surface. Acidic pond water
is also likely to be corrosive to exposed plastic and metal surfaces, causing
the pond water to become a toxic cocktail of contaminants.
What if the pH exceeds
9.0?
From experience, an excessively high pH is less extreme on
our koi than a low pH, but should still be avoided. While a drop in pH is
caused quite naturally, excessively alkaline water is likely to have been
caused by a pollutant, with cement or builder’s lime the No.1 suspect.
The
symptoms of an excessively alkaline pond will cause fish to secrete excess
mucus and also lead to gasping at the surface, very similar in fact to those
when fish experience acidic conditions.
A buffer is a chemical that when added to a pond, will help
to stabilise a suitable pH. The natural tendency is for a pond’s pH to become
acidic. A buffer reacts with any excess hydrogen ions (if and when an excess
arises) and will release them back again into the pond water should the pH rise
too high.
The most widely used buffer in and around ponds is calcium carbonate
(CaCO³). This can be in the form of crushed shells, limestone gravel or similar
material.
How a buffer works in
your pond
Let’s say, through extreme levels of respiration, carbon
dioxide is released into the water. This combines with water, to form carbonic
acid, which dissociates into two different ions.
Example: H²O + CO² H²CO³ H+ + H+ + CO³²-
The free hydrogen ions have a potential to make the water
acidic.
Adding a buffer – such as calcium carbonate (CaCO³) to ‘soak
up’ those free hydrogen ions, prevents the pH from dropping.
By adding limestone gravel to your filter, the following
will occur.
CaCO3 + H2CO3
Ca(HC03)2 Ca++ + HC03- + HC03-
Limestone + carbonic acid equals Calcium bicarbonate, which
dissociates into Calcium and bicarbonate ions (The hydrogen ions are no longer
‘free’ so do not contribute to the pond’s acidity).
This buffer will also work in reverse, releasing hydrogen
ions back into the water should the water become excessively alkaline. For
example, when the pH rises towards pH9, the calcium bicarbonate (which is now
dissolved in the water) will release its hydrogen ions, causing the pH to drop
slightly (but still remain alkaline), leading to a deposition of insoluble
calcium carbonate onto submerged surfaces – including some plants.
Note
Water is an excellent solvent. Any substance that become
dissolved in a solvent is called a solute. So salt (the solute) is dissolved in
water (the solvent) to make a salt solution. A solution, if sampled at any
number of points will be made up of precisely the same proportions of solute
and solvent.
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