Water quality, pH and fish health.
What is pH and why is it so important to fish health and water
quality? The simplest explanation is that it is a measurement of the
acidity or alkalinity of a substance – in this case pond water.
Changes in either can exert a powerful influence over both water
quality and water chemistry, and will have a marked effect on the fish
health and filter activity.
Although it determines the acidity or alkalinity of water, strictly
speaking we are actually measuring the quantity or ratio of two
ion (H+), responsible for acidity and
ion (OH-), which is responsible for
Some substances release hydrogen ions, or cause hydrogen ions to be
formed when they dissolve in water, while others release or create
The process of molecules splitting apart to form ions is called
ionisation. Fish keepers do not need to understand the mechanics of
ionisation, only to appreciate that it does happen – but if you have
any queries let me know. At any time water will contain both species of
ions with pH being a sort of balance sheet showing which is the
The pH scale
As we have already said, pH is determined by the relative quantities
of the hydroxyl and hydrogen ions. The pH scale measures these ratios on
a scale of 0 to 14.Very acidic solutions where the hydrogen ion
predominates are measured as 0 on the scale. Very alkaline solutions in
which hydroxyl ions predominate are 14 on the scale. At around pH 7,
depending on temperature and salinity, the numbers of both species
present are equal and therefore the water is neither acid or alkaline
– it is said to be neutral.
The pH of many common substances
The simplicity of the scale can be misleading and we should be aware
that the pH scale is a logarithmic
measurement of the concentration of hydrogen ions, which means that each
one unit change in the scale equals a ten-fold increase or decrease.
Therefore 8 is 10-times as alkaline as 7, while 9 is 100-times as
alkaline. So, these seemingly small changes actually represent major
changes in acidity or alkalinity.
Each one unit
increase in pH value represents a ten-fold increase in
Water hardness and pH
In pure water – that is water that contains very little in the way
of 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, lakes and the seas, for various
biochemical reasons, fish and other organisms would not survive.
Luckily such fluctuations are stabilized by the presence of
water-hardness causing substances. These molecules and ions act as a ‘buffers’
and ‘mop up’ any suddenly changes in the hydrogen/hydroxyl ratio.
The chemistry of this, for those that are interested, is explained more
fully on the water hardness page. However, as far as pH is concerned it
is important that it is considered in context with water hardness as the
two are closely related.
Water that is poorly buffered will be subject to more pH fluctuations
than well-buffered water. As a general rule, hard water is usually
alkaline and well buffered, whereas soft water is usually slightly
acidic and poorly buffered.
Water hardness and
pH are closely connected, with pH stability dependant on on the
buffering capacity of the water
What affects pH?
Many compounds added or dissolved in water will affect the pH by
adding or creating additional hydrogen or hydroxyl ions. Typically
cement or concrete will make water more alkaline. By far the biggest
influences are plant and animal respiration and plant photosynthesis.
All submerged plants and animals, including algae, are constantly
removing dissolved oxygen from the water and excreting carbon dioxide
during normal respiration. The release of carbon dioxide has an
acidifying effect. In addition to respiration, during daylight hours all
plants, which include all algal forms, actively photosynthesis. They
absorb carbon dioxide from the water and use the sun’s energy to
convert it to simple organic carbon compounds – clever!
As we have already said, 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 daylight hours plants have a net alkalising affect.
However, during the night plants stop photosynthesis but normal
respiration continues, so now they only remove oxygen from the water and
excrete carbon dioxide as part of normal respiration (so much for ‘oxygenating
plants’), with a net acidifying affect. In poorly buffered water this
can cause significant diurnal swings (over a period of 24-hours) in pH.
Even reasonably well buffered water may have a moderate variation in
pH during the day - being more alkaline in the evening. Therefore the
time of day that you take you reading is important. It is also advisable
to check the degree of fluctuation on a typical hot, sunny day.
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.
is the main cause of high pH and diurnal pH fluctuations
Fish health and pH
Each species of fish has its own very narrow range of pH
preference and levels outside of this range will cause health
problems. For example, koi prefer a range between 7 and 8.5, while
some tropical fish prefer water that is slightly acidic. There are
several ways that pH can affect fish 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 mucus production and encourage
epithelial hyperplasia (thickening of the skin or gill epithelia)
with sometimes-fatal consequences.
Fish 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 diurnal shifts in pH, mainly as a
consequence of photosynthesis as explained above. Large,
fluctuations – even though they may
still be within the preferred range – are likely to
be stressful and damaging to health.
As well as fish we should bear in mind that nitrifying bacteria
in the filter also have a narrow pH range preference between 7.5
Changes in pH will affect the toxicity of many dissolved
compounds. For example, ammonia becomes more toxic as pH
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 in at low pH, while potassium
permanganate is more dangerous at high pH. (See the treatment pages for
If you have large pH fluctuations, or levels are outside the
preferred range check the pH and hardness of you local water
supply. If this is within the preferred range, then partial water
changes over a period of time should redress the problem.
If you need to physically alter the pH, use a proprietary
buffer for aquaria. For ponds with a low pH – that is too
acidic- use agricultural lime (calcite CaCO3
or dolomite CaMg[CO3]2)
to buffer the water and increase pH. Do
not use slaked lime or unslaked lime. It needs to be added over a period of a
few days until the required level is reached. I would suggest some
experiments in 10 litres of water to establish the amount needed
for the whole pond (we are talking about several kilos normally).
For ponds that have a high pH – that is the water is too
alkaline, first check whether the temporary water hardness is OK.
This can be measured using a simple test kit for either kH,
carbonated hardness or alkalinity, which are all the same. If the
water is too soft use agricultural lime as detailed above. If on
the other hand the hardness is fine use either gypsum (plaster of
Paris) or calcium chloride. This will improve calcium, or general
hardness and stabilize pH. Substantial amounts may be needed. Some
experiments in 10 litres of water will give some indication, and
agricultural gypsum is available if you need large quantities.
Make the adjustments over a period of several days.