A Primer on Water Chemistry


Aquatic organisms – whether they are plants or animals – are influenced to a greater or lesser extent by the physical, chemical and electrical properties of the water in which they live. Sphagnum mosses, for example, grow in highland areas with poor drainage, igneous rocks and low average temperatures. In these conditions, the water is soft and slightly acid. Taken from their natural habitat and placed in hard alkaline water, sphagnum mosses quickly die. Fortunately, most aquatic  plants – including those used as aquarium subjects – are very adaptable to a wide variety of water conditions.

To grow more sensitive plants successfully in an aquarium, the water quality must closely simulate that of their natural environment. In addition to the water temperature and its ‘cleanliness’, the most important aspects of water quality as far as plants are concerned are its hardness and its degree of acidity or alkalinity. Here we briefly consider these parameters and how they are measured.

H2CO3 Molecule

Water hardness

To understand how water becomes hard or soft, we must break into the natural water cycle as rainwater falls through the atmosphere. On its downward journey, rain reacts with small amounts of carbon dioxide to produce carbonic acid, as shown below;

            CO2 + H2O ——-> H2CO3

This simple reaction is the key to many changes that occur when rainwater becomes groundwater. As we have seen in our example of sphagnum mosses, where rainwater falls on hard, igneous (and therefore, inert) rocks the groundwater remains low in dissolved mineral salts – i.e. ‘soft’ and slightly acidic. (In fact, organic acids from dead and decaying plants also add to the acidity of the water.) But where the slightly acidic rainwater flows through sedimentary rocks, such as chalk and limestone, further chemical reactions occur that add ‘hardness’ to the water. The basic reaction involves the action of carbonic acid on calcium carbonate (chalk) to produce calcium bicarbonate, as shown below:

            H2CO3 + CaCO3  ——–> Ca(HCO3)2

Equivalent reactions involve magnesium carbonate.

Since the bicarbonates produced can be easily broken down by boiling, this type of hardness is called temporary hardness or carbonate hardness. (It is normally designated as KH.) As the bicarbonates decompose, the carbonate portion is deposited as white scale – a familiar sight inside kettles for people living in hard water areas. (The level of bicarbonates in water is also important for aquarists because they act as a ‘storage area’ for carbon dioxide and help to stablilize the acidity/alkalinity balance of the water.)

But calcium and magnesium carbonates are not the only mineral salts involved in producing water hardness. Reactions with other naturally occurring compounds in rocks produce a veritable ‘cocktail’ of mineral salts, so that hard water may contain the sulphates, carbonates, bicarbonates, nitrates and chlorides of calcium, magnesium, barium and strontium. The level of hardness produced by the sum total of these substances is called, quite logically, the total or general hardness (GH).

Water hardness is measured in a confusing range of units. In this book we have used the German scale of odH, which can be applied either to general hardness or to carbonate hardness. As far as aquarium plants are concerned, carbonated hardness is the most significant of the two parameters. Alternatively, general hardness can be expressed in terms of miligrams per litre of calcium carbonate (mg/l CaCO3).

For convenience, the table shows the comparative levels of hardness in both these scales.

odHMg/litre CaCO3Considered as
3-650-100Moderately soft
6-12100-200Slightly hard
12-18200-300Moderately hard
Over 25Over 450Very hard

The fraction of general hardness that cannot be removed by boiling is called permanent hardness and is caused mainly by calcium sulphate.

Most of our tropical aquarium plants come from areas of soft water, whereas some from subtropical areas grow in relatively hard water. In practice, most plants adapt very well to growing in domestic tapwater, which is invariably hardened to reduce corrosion in metal pipes. Where this is the case, we have used the term ‘not critical’ against the recommended hardness level for each plant.

Various test kits are widely available for measuring the hardness of water. Most involve counting the number of drops of a reagent needed to change the colour of an indicator added to a measured water sample. Electronic meters that register hardness by measuring the electrical conductivity of the water give accurate and speedy results, but are relatively expensive and delicate pieces of equipment for the aquarist.

Acidity and alkalinity

Describing water as either ‘acidic’ or ‘alkaline’ is relative in terms of a ‘neutral’ point. The universally accepted way of expressing this aspect of water quality is in terms of pH value. The pH scale is a logarithmic calibration based inversely on the concentration of hydrogen ions in the water. Thus, the more hydrogen ions there are in the water the greater is its acidity and the lower its rating on the pH scale. On the scale, pH 7 is neutral, with values from 7 down to 0 denoting increasing acidity and those from 7 to 14 signifying increasing alkalinity. The logarithmic nature of the scale means that one whole number step represents a ten-fold variation, two steps represent a hundred-fold variation and three steps a thousand-fold variation, etc.

Aquarium plants are not as drastically affected by violent changes in pH values as are most fishes, although some authorities suggest that so-called ‘cyptocoryne rot’ is caused by rapid variations in pH level. Most aquarium plants will thrive in water with a pH value in the range 6.5-7.4. 

As for water hardness, easy-to-use test kits are available for measuring pH value. These include kits using liquid reagents as well as paper strips impregnated with suitable indicators.