From
http://srac.tamu.edu/index.cfm?catid=25Its about the Ca and Mg salts which may not necessarly be there if the alkalinity is from sodium bicarbonate.
Interactions of pH, Carbon Dioxide,
Alkalinity and Hardness in Fish Ponds
Hardness
Water hardness is important to
fish culture and is a commonly reported
aspect of water quality. It
is a measure of the quantity of divalent
ions (for this discussion,
salts with two positive charges)
such as calcium, magnesium
and/or iron in water. Hardness
can be a mixture of divalent salts;
however, calcium and magnesium
are the most common sources of
water hardness.
Hardness is traditionally measured
by chemical titration. The
hardness of a water sample is reported
in milligrams per liter as
calcium carbonate (mg/L CaCO3).
Calcium carbonate hardness is a
general term that indicates the
total quantity of divalent salts present
and does not specifically identify
whether calcium, magnesium
and/or some other divalent salt is
causing water hardness.Hardness is commonly confused
with alkalinity (the total concentration
of base). The confusion relates
to the term used to report
both measures, mg/L CaCO3.
If
limestone is responsible for both
hardness and alkalinity, the concentrations
will be similar if not
identical. However, where sodium
bicarbonate (NaHCO3) is responsible
for alkalinity it is possible
to have low hardness and high
alkalinity. Acidic, ground or well
water can have low or high hardness
and has little or no alkalinity.Calcium and magnesium are essential
in the biological processes
of fish (bone and scale formation,
blood clotting and other metabolic
reactions). Fish can absorb calcium
and magnesium directly
from the water or from food.
However, calcium is the most important
environmental, divalent
salt in fish culture water. The presence
of free (ionic), calcium in culture
water helps reduce the loss of
other salts (e.g., sodium and potassium)
from fish body fluids (i.e.,
blood). Sodium and potassium
are the most important salts in fish
blood and are critical for normal
heart, nerve and muscle function.
Research has shown that environmental
calcium is also required to
re-absorb these lost salts. In low
calcium water, fish can lose (leak)
substantial quantities of sodium
and potassium into the water.
Body energy is used to re-absorb
the lost salts. For some species
(e.g., red drum and striped bass),
relatively high concentrations of
calcium hardness are required for
survival.
A recommended range for free calcium
in culture waters is 25 to 100
mg/L (63 to 250 mg/L CaCO3
hardness). Channel catfish can tolerate
low calcium concentrations
A low CaCO3 hardness value is a
reliable indication that the calcium
concentration is low.
However,
high hardness does not necessarily
reflect a high calcium concentration.But, since limestone is common
in the soil and bedrock of the
southern United States, it would
be reasonably safe to assume that
high hardness measurements reflect
high calcium levels.
A CaCO3 hardness value of 100
mg/L represents a free calcium
concentration of 40 mg/L (divide
CaCO3 value by 2.5) if hardness is
caused by the presence of calcium
only. Similarly, a CaCO3 value of
100 mg/L represents a free magnesium
value of 24 mg/L (divide
CaCO3 value by 4.12) if hardness
is caused by magnesium only.
These factors (2.5 and 4.12) are related
to the molecular weight of
CaCO3 and the difference in
weights between calcium and magnesium
atoms.
Where hardness is
caused by limestone, the CaCO3
value usually reflects a mixture of
free calcium and magnesium with
calcium being the predominant divalent
salt.Ideally, an aquaculture pond
should have a pH between 6.5 and
9 as well as moderate to high total
alkalinity (75 to 200, but not less
than 20 mg/L) and a calcium hardness
of 100 to 250 mg/L CaCO3.Many of the principles of chemistry
are abstract (e.g., carbonate-bicarbonate
buffering) and difficult
to grasp. However, a fundamental
understanding of the concepts and
chemistry underlying the interactions
of pH, CO2, alkalinity and
hardness is necessary for effective
and profitable pond management.
There is no way to avoid it; water
quality is water chemistry.