Water hardness is, from a fish’s viewpoint, one of the most
important aspects of water quality as it affects so many areas of fish
health. Despite this importance to both fish health and water quality,
it is often a poorly understood subject. Indeed, many fish keepers never
check water hardness. This seems particularly true of koi keepers to
whom it is especially relevant because of the high levels of
nitrification taking place in a well-stocked koi pond.
Water hardness has a major effect on pH and pH stability. It will
affect the toxicity of many common substances, including some fish
disease treatments. It also has a major effect on fish osmoregulation, a process you will
recall that is vital to fish health.
So what is water hardness?
As explained in “The Birth of Water”,
water accumulates many dissolved substances before it reaches our taps.
Hardness is a measurement of the concentration of divalent metal ions
such as calcium, magnesium, iron, zinc etc, usually acquired as
rainwater percolates through rock. In most water it consist mainly of
calcium and magnesium salts, with trace amounts of other metals.
Two types of hardness:
The subject gets a little confusing because there are two types of
hardness that we need to consider. The two types are permanent hardness and alkalinity (often referred to as carbonate or
temporary hardness). The sum of both types of hardness is called
the general or total hardness.
Alkalinity refers to the hardness
derived mainly from carbonate and bicarbonate ions and directly reflects
the buffering capacity of the water. This form of hardness is also
called carbonate hardness or temporary hardness because it can be
precipitated and removed by boiling the water. Which is why lime-scale
forms in kettles and showerheads!
Permanent hardness measures the
ions such as nitrates, sulphates, and chlorides etc, that are not
removed by boiling. Most of these are not involved with buffering but
can affect pH values.
In most water supplies general hardness
and alkalinity measurements (as
mg/litre CaCO3) are likely to be very similar
because carbonates usually predominate and the amount of permanent
hardness is usually fairly small.
While there is a very close connection between water hardness and
buffering it should be made clear that hardness is a product of mainly
calcium and magnesium ions, while buffering is produced by bicarbonate
and carbonate ions. The fact that the two are so closely related is due
to the fact that most hardness is formed from calcium and magnesium
So, as a rule of thumb, hard water is usually well buffered while
soft water is usually less well buffered. However, we should be aware
that it is possible, because of different water composition, to have
hard water that is poorly buffered, i.e water where permanent hardness
predominates, or soft water that is well buffered, i.e. water that has
high levels of sodium or potassium carbonate, rather than calcium or
magnesium. Obviously the simple way to establish the makeup of your
local water and pond water (they may not be the same) is to test
for both types of hardness. Test kits are readily available for
measuring both types of hardness.
The carbon dioxide/ bicarbonate/ carbonate buffering
The initial pH of water is determined by the type of dissolved
compounds that it accumulates, although it may well be chemically
altered by the water company before it reaches your tap. However, once
it is in the pond or aquaria, water pH is also influenced by other
factors such as plant and animal respiration and plant photosynthesis. Without some
form of buffering these natural activities would cause huge diurnal
swings in pH.
The most common buffering system is the the carbon dioxide/
bicarbonate/ carbonate buffering system. Essentially it stabilises pH by
mopping up excess hydrogen ions and then releases them again as levels
drop, so that the hydrogen concentration, and therefore the pH, stays
+ H 2O H2CO3 HCO
- + H + CO32-
(solid) + 2H +
What this equation tells us (from left to right) is that carbon
dioxide, excreted by fish and plants, dissolves in water to form
carbonic acid (H2CO). If pH levels increase,
that is the water becomes more alkaline (say from plant photosynthesis),
then the carbonic acid dissociates to form bicarbonate and hydrogen ions
(HCO3- + H+). Hydrogen ions are
acidic-forming ions and will therefore counteract the alkalinity
increase. If the pH continues to increase, the bicarbonate will
dissociate to form solid carbonate and release yet more hydrogen ions
(CO32- (solid) + 2H+), to counteract
the increased alkalinity. The solid carbonate is the chalk layer
covering the pond bottom and walls (or the kitchen kettle). If pH levels
start to fall the process is reversed. At a normal pond pH of 7-8 some
of all of the above species will be present, with bicarbonate
dominating. Carbonate will predominate above pH 9.
The buffering capacity of water depends on the total amount of
bicarbonate and carbonate present. Water that has low levels of these
ions will quickly exhaust its ability to counteract pH fluctuations.
How much hardness?
Each fish species has its preferred range of water hardness, however
it can become confusing as there are several units of measurement
currently used to determine water hardness. The most commonly used
method measures both alkalinity and general hardness as mg /litre of
calcium carbonate (CaCO3). Another common measurement, used
by Tetra, is German hardness measured as odH. These compare
as seen in table 1.
mg / litre
||0o - 4o
||75 - 150
||4o - 8o
||150 - 300
||8o - 16o
|to convert odH to CacO3
multiply by 17.9
Table 1: Typical water hardness ranges
Fish health and water hardness:
Different species of fish have varied water hardness requirements, so
it is important to find out what hardness is best for your fish, For
most pond fish, i.e. koi and goldfish, moderate to hard water is best.
From table 1 above we can see that the optimum hardness range for most
pond fish would be between 100 - 300 mg/litre CaCO3
Water hardness affects fish health because it influences
osmoregulation. Being open systems, fish are affected by the makeup of
the surrounding water. As a consequence of osmosis, freshwater fish are
subject to a continuous influx of water, while marine fish have to live
with a continuous outflow of water.
Against this continuous movement of water into or out of the body,
fish have to maintain a constant internal body fluid concentration – a
process called osmoregulation. The
greater the difference in concentration between the fish’s body fluids
and the surrounding water – the greater the osmotic effect. As hard
water is more concentrated than soft, there will be less difference and
therefore less water influx and consequently the fish will not have to
work so hard at osmoregulation. This is particularly important in cases
of bacterial ulceration where water can flood into open tissues.
Water hardness and disease treatments:
Some common fish disease treatments are affected by water hardness,
and therefore needs to be considered when calculating dosages. Probably
the most sensitive is chloramine-T, which is quite toxic in soft, acidic
How do I change water hardness?
First because nitrification is continually removing alkalinity it is
important that hardness is monitored on a regular basis – say about
once a month. If either alkalinity or general hardness falls below the
optimum level it can be reversed by either adding a calciferous source
such as crushed oyster shell to the filter or adding more buffer to the
water. If alkalinity is too low then add a carbonate buffer. If general
hardness is too low then add a calcium or magnesium buffer.