Alkalinity refers to the total
amount of bases in water
expressed in mg/l of equivalent
calcium carbonate. A base is a
substance that releases hydroxyl ions (OH-) when dissolved in water. In most waters these bases are
principally bicarbonate (HCO) ions and carbonate ions (CO32-). These ions are the buffers in water; that
is they buffer the water against
sudden changes in pH. They can do
this by absorbing hydrogen ions
when the water is acid and
releasing them when the water becomes basic. Waters of low
alkalinity ( buffered, and the removal of
carbon dioxide (CO2) during photosynthesis results in rapidly
rising pH. Waters, with greater
than 20 mg/l alkalinity have
greater buffering capacity and
prevent large fluctuations in pH
during photosynthesis (Fig. 1). Fig. 1. The effect of buffering on pH at low and high alkalinities Hardness is the concentration of
metal ions (primarily calcium and
magnesium) expressed in mg/l of
equivalent calcium carbonate.
Alkalinity and hardness values are
normally similar to magnitude because calcium, magnesium,
bicarbonate, and carbonate ions in
water are derived in equivalent
quantities from the solution of
limestone in geological deposits.
However, in some waters alkalinity may exceed its hardness and vice
versa. If alkalinity is high and
hardness low, pH may rise to very
high levels (greater than 10.5)
during periods of rapid
photosynthesis. Waters are often categorised
according to degrees of hardness
as follows: 0-75 mg/l – soft 75-150 mg/l – moderately hard 150-300 mg/l – hard over 300 mg/l – very hard Alkalinity and hardness are not
greatly affected by biological
activity or aquacultural operations,
and the initial concentrations in
ponds are determined by their
level in the water supply; any changes are largely the result of
rainfall and evaporation. Desirable
levels for fish culture generally fall
within the range of 20-300 mg/l. If
total alkalinity and total hardness
are too low, they may be raised by liming. However, there is no
practical way of decreasing
alkalinity and hardness when they
are above desirable levels. As a general rule, the most
productive waters for fish culture
have a hardness and alkalinity of
approximately the same
magnitude. For example, a water
with an alkalinity of 100 mg/l and hardness of 10 mg/l is not as good
for fish culture as water in which
the alkalinity is 100 mg/l and the
hardness is 100 mg/l. Greater
production does not result directly
from higher levels of hardness and alkalinity per se, but from the
higher concentrations of
phosphorus and other essential
elements that increase along with
hardness and alkalinity.
amount of bases in water
expressed in mg/l of equivalent
calcium carbonate. A base is a
substance that releases hydroxyl ions (OH-) when dissolved in water. In most waters these bases are
principally bicarbonate (HCO) ions and carbonate ions (CO32-). These ions are the buffers in water; that
is they buffer the water against
sudden changes in pH. They can do
this by absorbing hydrogen ions
when the water is acid and
releasing them when the water becomes basic. Waters of low
alkalinity ( buffered, and the removal of
carbon dioxide (CO2) during photosynthesis results in rapidly
rising pH. Waters, with greater
than 20 mg/l alkalinity have
greater buffering capacity and
prevent large fluctuations in pH
during photosynthesis (Fig. 1). Fig. 1. The effect of buffering on pH at low and high alkalinities Hardness is the concentration of
metal ions (primarily calcium and
magnesium) expressed in mg/l of
equivalent calcium carbonate.
Alkalinity and hardness values are
normally similar to magnitude because calcium, magnesium,
bicarbonate, and carbonate ions in
water are derived in equivalent
quantities from the solution of
limestone in geological deposits.
However, in some waters alkalinity may exceed its hardness and vice
versa. If alkalinity is high and
hardness low, pH may rise to very
high levels (greater than 10.5)
during periods of rapid
photosynthesis. Waters are often categorised
according to degrees of hardness
as follows: 0-75 mg/l – soft 75-150 mg/l – moderately hard 150-300 mg/l – hard over 300 mg/l – very hard Alkalinity and hardness are not
greatly affected by biological
activity or aquacultural operations,
and the initial concentrations in
ponds are determined by their
level in the water supply; any changes are largely the result of
rainfall and evaporation. Desirable
levels for fish culture generally fall
within the range of 20-300 mg/l. If
total alkalinity and total hardness
are too low, they may be raised by liming. However, there is no
practical way of decreasing
alkalinity and hardness when they
are above desirable levels. As a general rule, the most
productive waters for fish culture
have a hardness and alkalinity of
approximately the same
magnitude. For example, a water
with an alkalinity of 100 mg/l and hardness of 10 mg/l is not as good
for fish culture as water in which
the alkalinity is 100 mg/l and the
hardness is 100 mg/l. Greater
production does not result directly
from higher levels of hardness and alkalinity per se, but from the
higher concentrations of
phosphorus and other essential
elements that increase along with
hardness and alkalinity.
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