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Water Hardness


  1. CO2, carbonate hardness, etc.
    by booth-at-hplvec.LVLD.HP.COM (George Booth) (Tue, 6 Apr 1993)

CO2, carbonate hardness, etc.

by booth-at-hplvec.LVLD.HP.COM (George Booth)
Date: Tue, 6 Apr 1993

This should make it to the FAQ sometime.  Does this help?



  Water hardness is of interest to aquarists for two reasons: to
provide the proper environment for the fish and to help stabilize the
pH in the aquarium.  There are two types of water hardness: general
hardness (GH) and carbonate hardness (KH).  A third term commonly used
is total hardness which is a combination of GH and KH.  Since it is
important to know both the GH and KH, the use of total hardness can be
misleading and should be avoided.


  General hardness is primarily the measure of calcium (Ca++) and
magnesium (Mg++) ions in the water.  Other ions can contribute to GH
but their effects are usually insignificant and the other ions are
difficult to measure.  GH will not directly affect pH although "hard"
water is generally alkaline due to some interaction of GH and KH.

  GH is commonly expressed in parts per million (ppm) of calcium
carbonate (CaCO3), degrees hardness (dH) or, more properly, the molar
concentration of CaCO3.  One German degree hardness (dH) is 10 mg of
calcium oxide (CaO) per liter.  In the U.S., hardness is usually
measured in ppm of CaCO3.  A German dH is 17.8 ppm CaCO3.  A molar
concentration of 1 milliequivalent per liter (mEq/l) = 2.8 dH = 50
ppm.  Note that most test kits give the hardness in units of CaCO3;
this means the hardness is equivalent to that much CaCO3 in water but
does not mean it actually came from CaCO3.  Water hardness follows
these guidelines:

      0 -  4 dH,    0 -   70 ppm : very soft
      4 -  8 dH,   70 -  140 ppm : soft
      8 - 12 dH,  140 -  210 ppm : medium hard 
     12 - 18 dH,  210 -  320 ppm : fairly hard
     18 - 30 dH,  320 -  530 ppm : hard
     higher : liquid rock (Lake Malawi and Los Angeles, CA)

  General hardness is the more important of the two in biological
processes.  When a fish or plant is said to prefer "hard" or "soft"
water, this is referring to GH.  Incorrect GH will affect the transfer
of nutrients and waste products through cell membranes and can affect
egg fertility, proper functioning of internal organs such as kidneys
and growth.  Within reason, most fish and plants can successfully
adapt to local GH conditions, although breeding may be impaired.


  Carbonate hardness (KH) is the measure of bicarbonate (HCO3-) and
carbonate (CO3--) ions in the water.  In freshwater aquariums of
neutral pH, bicarbonate ions predominate and in saltwater aquariums,
carbonate ions begin to play a role.  Alkalinity is the measure of the
total acid binding capacity (all the anions which can bind with free
H+) but is comprised mostly of carbonate hardness in freshwater
systems.  Thus, in practical freshwater usage, the terms carboante
hardness, acid binding, acid buffering capacity and alkalinity are
used interchangeably.  In an aquarium, KH acts as a chemical buffering
agent, helping to stabilize pH.  KH is generaly referred to in degrees
hardness and is expressed in CaCO3 equivalents just like GH.

  In simple terms, pH is determined by the negative log of the
concentration of free hydrogen ions (H+) in the water.  If you add a
strong acid such as nitric acid to water, it completely dissociates
into hydrogen ions (H+) and its "conjugate base" or "salt", NO3- or
nitrate.  The hydrogen ions freed in the reaction then increase the
concentration of hydrogen ions and reduce the pH.  Since nitric acid
is the end product of the nitrogen cycle, this explains why aquarium
pH tends to decrease and nitrates tend to increase over time.

  When the aquarium has some carbonate buffering in it, the
bicarbonate ions will combine with the excess hydrogen ions to form
carbonic acid (H2CO3) which then slowly breaks down into CO2 and
water.  Since the excess hydrogen ions are used in the reaction, the
pH does not change very much.  Over time, as the carbonate ions are
used up, the buffering capacity will drop and larger pH changes will
be noted.  From this it is clear why aquariums with low KH seem
unstable - as acid is produced by biological action, the KH is used
up; when it is gone, the pH is free to drop rapidly as H+ ions are


  If your local water is too hard for the fish and plants you desire,
it can be softened.  There are many ways to do this but some are more
suited to aquarium use than others.  The best (and most expensive, of
course) is to use a reverse osmosis (RO) deionizer and mix the
resulting water (GH=0) with tap water to get the desired GH.  Peat
moss can be used to soften and condition the water for use in South
American cichlid tanks, but will add a slight tea color to the water.
Peat filtering may be difficult to control.  Peat should be boiled
first to kill any unwanted organisms. 

  Commerical water softening resin "pillows" can be used on a small
scale, but are not effective for larger amounts of water.  Water
softening systems designed for large scale home use (like bath water)
are not suitable since they use an ion exchange principle: usually
sodium ions are substituted for calcium and magnesium ions and excess
sodium is not desired in the aquarium.  An even worse practice is to
use a cation exchange resin in the hydrogen ion form and use it to
pull divalent ions out of the water.

  If the local GH is too low, it can be raised by adding calcium
sulfate and/or magnesium sulfate.  This has the drawback of
introducing sulfates (SO4--) into the water, so care should be
exercised.  Calcium carbonate can be used, but it will also raise the
KH (this is ideal for the lucky few who have naturally soft water).
Various combinations can be used to produce the desired results.

  Carbonate hardness can be reduced by boiling the water (inpractical
for all but the smallest aquariums; let it cool before adding to the
tank :-) or by peat filtering.

  Carbonate hardness can be easily increased by adding sodium
bicarbonate.  Calcium carbonate will increase both KH and GH in equal

  One teaspoon (about 6 grams) of sodium bicarbonate (NaHCO3) per 50
liters of water will increase KH by 4 degrees and will not increase
general hardness.  Two teaspoons (about 4 grams) of calcium carbonate
(CaCO3) per 50 liters of water will increase both KH and GH by 4
degrees.  Different proportions of each can be used to get the correct
KH/GH balance dictated by the fish and plants in the tank.  Since it
is difficult to accurately measure small quantities of dry chemicals
at home, a test kit should be used to verify the actual KH and GH that
is achieved.


  In more detail, the pH of a buffered solution can be expressed by the
Henderson-Hasselbach equation:

  pH = pK + log ----  

where pK is one or more "equilibrium dissociation constants" of the
weak acid.  In the bicarbonate and carbonate buffering cases, this is:

                  HCO3-                            CO3--
  pH = 6.37 + log -----    and    pH = 10.25 + log -----          
                  H2CO3                            HCO3-
Note that the pK values are affected rather appreciably by temperature
and chlorinity.  If you plot the pH versus the ratio of base to acid,
you will get a logarithmic graph something like this:

 100% base             ,---------      ,----------
                      /               /
              H2CO3  |     HCO3-     |      CO3--
  50% mix     + CO2  |<- pH=6.37     |<- pH=10.25
                     |               |
                     /               /    
 100% acid  --------`       --------`    

       pH   ...5...6...7...8...9...10..11..12...

Bicarbonate buffering is effective over ratios from 1:100 up to
100:1.  This gives an effective pH range of 4.37 to 8.37, which, not
coincidently, defines the pH range of most aquatic life.
  If you add bicarbonate ions (for example, by adding sodium
bicarbonate or calcium carbonate), the base to acid ratio will
increase and the pH will increase.  From the graph, the rate of
increase will be determined by the pH you started with: at pH = 6.37,
you will need a lot of bicarbonate ions to change the pH; at pH=7.5,
you will need a lot less.  (Note: the chemical equilibria of the
various components of the carbonate system (CO2, H2CO3, HCO3- and
CO3--) are very complex and are beyond the capability of the author to
fully describe).

  The rise in pH that occurs when KH is added will be balanced to a
degree by the dissolved CO2 in the water.  Fortunately, CO2 is also a
result of the nitrification process and fish and plant respiration so
it is readily available.  The CO2 will form small amounts of carbonic
acid and bicarbonate which will tend to reduce the pH.  This mechanism
gives us a way to regulate pH in the aquarium.

  If the pH of an aquarium is determined PRIMARILY by the carbonate
buffering system, then the relation of pH and KH and dissolved CO2 is
fixed.  You can change either KH or CO2 to set the pH.  An automatic CO2
injection system will measure pH and inject CO2 to lower it if it
exceeds a set point.  In this case KH is fixed.  As the CO2 is used by
plants and diffuses into the atmosphere, the pH will rise.  The
controller cycles the CO2 on and off to keep the pH around a fixed

  The following chart shows dissolved CO2 levels in ppm for a range of
KH and pH values:

                 degrees KH
            2    3    4    5    6
     6.6 | 15   23   30   38   46
     6.7 | 12   18   24   30   36
     6.8 |  9   14   19   24   28
 pH  6.9 |  7   12   15   19   23
     7.0 |  6    9   12   15   18
     7.1 |  5    7    9   12   14
     7.2 |  4    6    8    9   11
     7.3 |  3    4    6    7    9
     7.4 | 2.4   3    5    6    7
     7.5 | 1.9  2.5  3.5   5    5
     7.6 | 1.5   2   2.5   3    4
     7.7 |  1   1.5   2   2.5   3
     7.8 | 0.9  1.1  1.5   2    2
     7.9 | 0.6  0.9   1   1.2  1.6
     8.0 | 0.5  0.7  0.9   1   1.2

Note that typical dissolved CO2 levels in a moderately stocked
aquarium will be in the range of 2-3 ppm.  From the chart, it is clear
that almost any carbonate hardness will produce a pH in the mid 7
range unless extra CO2 is added.  For a typical planted aquarium,
pH=6.9, KH=4 and CO2=15 ppm is just about ideal.

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This page was last updated 29 October 1998