Water Chemistry

Copyright 2000 by George and Karla Booth

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

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

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 generally 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 generated.

Adjusting Freshwater Hardness

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.

Commercial 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 (impractical 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 parts.

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.

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. 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 scope of this book).

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 value.

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

 \ pH | 6.0     6.2     6.4     6.6     6.8    7.0    7.2    7.4    8.0
KH\   |
0.5   | 15      9.3     5.9     3.7     2.4    1.5    0.9    0.6    0.2
1.0   | 30      19      12      7       5      3      1.9    1.2    0.3
1.5   | 44      28      18      11      7      4      2.8    1.8    0.4
2.0   | 59      37      24      15      9      6      4      2.4    0.6
2.5   | 73      46      30      19      12     7      5      3      0.7
3.0   | 87      56      35      22      14     9      6      4      0.9
3.5   | 103     65      41      26      16     10     7      4      1.0
4.0   | 118     75      47      30      19     12     6      5      1.2
5.0   | 147     93      59      37      23     15     9      6      1.5
6.0   | 177     112     71      45      28     18     11     7      1.8
8.0   | 240     149     94      59      37     24     15     9      2.4
10    | 300     186     118     74      47     30     19     12     3
15    | 440     280     176     111     70     44     28     18     4
      |                 CO2  milligrams/liter
>From a Finnish aquaria magazine (Akvaariomaailma)