Although it determines
the acidity or alkalinity of water, strictly speaking we are
actually measuring the quantity or ratio of two important molecules;
ion (H+) and hydroxyl ion (OH-), which are responsible for acidity
Some substances release
H+ or caused H+ to be formed when they dissolve in
water, while others release or create H-.
The process of molecules
splitting apart to form ions is called ionisation. At any
time, water will contain both specied of ions with pH being a sort
of balance sheet
showing which is the predominate ion.
The pH scale measures
the rations of the relative quantities of H+ and H - on a
scale of 0 to 14. Very acidic solutions where the H+ predominates
as 0 on the scale and very alkaline solutions in which H - predominates
are 14 on
the scale. At around pH 7, depending on temperature and salinity,
of both species present are equal and therefore the water is neither
alkaline - it is said to be neutral.
The pH scale is a logarithmic
measurement of the concentration of H+, which
means that each one unit change in the scale equals a ten-fold increase
Thus, 8 is 10-times as alkaline as 7, while 9 is 100-times as alkaline.
seemingly small changes actually represent major changes in acidity
In water that contains
very little dissolved substances - the addition of very small
amounts of acid or alkaline substances can cause quite dramatic
shifts in pH.
If such wide swings were to occur in ponds, for various biochemical
and other organisms would not survive.
However, such fluctuations
are stabilized by the presence of water-hardness causing
substances. These molecules and ions act as a 'buffer' and 'mop
up' any sudden
changes in the H+ and H- ratio. Water hardness and pH are therefore
with pH stability dependent on the buffering capacity of the water.
As a general rule,
hard water is usually alkaline and well buffered, whereas soft water
is usually slightly
acidic and poorly buffered
Many compounds added
or dissolved in water will affect the pH by adding or creating
additional H+ and H-. Typically cement or concrete will make water
By far the biggest influences are plant and animal respiration and
All submerged plants
and animals, including algae, are constantly removing dissolved
oxygen from the water and excreting carbon dioxide during normal
release of carbon dioxide has an acidifying effect. In addition
to respiration, during
daylight hours all plants, which include algae forms, actively photosynthesis.
absorb carbon dioxide from the water and use the sun's energy to
convert it to simple
organic carbon compounds. As carbon dioxide in solution is slightly
acidic, so as the
plants remove it, the water becomes more alkaline. The more sunshine
and algae -
the more alkaline the water will become.
These two processes,
respiration and photosynthesis, carry on alongside each other,
with photosynthesis being the dominant during the day. Thus during
the day, plants have
a net alkalising effect. However, during the night, plants stop
photosynthesis but normal
respiration continues, so now they only remove oxygen from the water
carbon dioxide as part of normal respiration, with a net acidifying
effect. In poorly buffered
water this can cause significant pH swings.
The other significant
factor affecting pH is nitrification, which tends to have a slight
tendency to acidify water as well as removing the 'buffering' capacity
or hardness of water.
pH can affect your koi.
Koi prefer a range between
7 and 8.5, and levels outside of this range can cause health
problems. There are several ways that pH can affect koi health.
High acidity or alkalinity
can cause direct physical damage to skin, gills and eyes.
Prolonged exposure to sub-lethal pH levels can cause stress, increase
and encourage thickening of the skin or gill epithelia with sometimes-fatal
Koi also have to maintain
their own constant internal pH. Even small fluctuations of blood
pH can prove fatal. Extreme external or water pH can influence and
affect blood pH,
resulting in either acidosis or alkalosis of the blood.
The other consideration
is pH swing. Large fluctuations - even though they may still be
the preferred range - are likely to be stressful and damaging to
As well as koi, we should
bear in mind that nitrifying bacteria in the filter also have a
pH range preference between 7.5 and 8.6.
Changes in pH will affect
the toxicity of many dissolved compounds. For example, ammonia
becomes more toxic as pH increases.
Variances in pH will
also exert an effect on some common disease treatments, so it is
important to take account of pH (and usually water hardness) when
using treatments. For
example, chloramine-T is more toxic at low pH, while potassium permanganate
dangerous at high pH.