by Andrea Watts
People are often confused by the terms: ‘general hardness’, ‘total hardness’, ‘temporary hardness’, 'permanent hardness’, ‘carbonate hardness’ and ‘total dissolved salts’. The aim of this article is to explain the differences between them and give you a better understanding of water hardness.
Hardness is the measure of dissolved mineral salts (mainly chlorides, bicarbonates, carbonates, and sulphates of calcium, sodium, magnesium and potassium); the harder the water the more salts it contains. It is generally expressed in terms of calcium carbonate (CaCO3) content, measured in degrees (ºdH) or parts per million (ppm) using a test kit. Care is needed with regard to “degrees of hardness”, the definition of which varies from country to country and test kit to test kit, depending on origin.
Water becomes hard by dissolving soluble salts from the rocks or soil over or through which it flows. Some rocks, for example slate, granite, gneiss and schist, contain little or no soluble material and, as such, have a negligible effect. Others, however, are quite the opposite. Rocks and other “hard” décor in the aquarium may affect water chemistry: corals and shells are largely calcium carbonate, and some gravel often contains fragments of limestone or shell. Hardness-free décor is a pre-requisite of soft-water aquariums.
[ NB: to convert ºdgH / ºdkH (German degrees) to ppm, multiply ºdgH x 17.9. To convert to American degrees, multiply by 17.1. The English conversion (Clark) involves multiplying by 14.3 ]
Following is a general guide to fish suitability to water hardness parameters:
|gH and kH Range||Suitable Fish|
|0º - 3º (0 - 50 ppm)||arowanas, elephant nose, killifish, hatchet fish, Amazon/South American fish, including tetras, dwarf cichlids, geophagines, Ancistrus spp.|
|3º - 6º (50 - 100 ppm)||most tropical fish including Central American cichlids, East African Riverine species, gouramis, sharks, loaches, barbs, rainbows, African tetras|
|6º - 11º (100 - 200 ppm)||most live bearers including guppies, mollies, swordtails and platies, goldfish, Lake Victorian cichlids|
|11º - 22º (200 - 400 ppm)||Lake Malawi and Lake Tanganyikan cichlids, brackish fish including scats, monos and puffers|
There are many fish that are tolerant to a wide range of water hardness values. Some of these include many plecos, most Central American and East African species, gouramis, sharks, tiger barbs, widow tetras, most Australian rainbowfish, siamese fighting fish and most Corydoras species.
Total, general or permanent hardness is represented throughout literature by the letters gH. It is determined by the concentration of calcium and magnesium salts, i.e. the amount of calcium (Ca++) and magnesium (Mg++) ions, which are dissolved in the water. These minerals are present in town, artesian (bore) and bottled water. The amount of dissolved minerals is dependent upon the source of the water, and the type of treatment processes it has undergone. Total hardness or general hardness is sometimes referred to as total dissolved salts (TDS). Scientists often measure mineral content in terms not or hardness but of electrical conductivity, the units employed being micro-siemens (µS).
Hard water (> 200 ppm) is high in calcium and magnesium, while soft water (50 to 100 ppm) is low in these minerals. Suitable values for most ornamental fish range between 100 and 300 ppm. As an aquarist, you should seek to provide a healthy aquarium environment by duplicating the water conditions of the natural habitats of your fish. In a home aquarium, select fish within the same region so that the gH values are relatively similar.
Carbonate, bicarbonate or temporary hardness is formed from the compounds of calcium and magnesium with carbonic acid, i.e. it is the measure of carbonate (CO3-) and bicarbonate (HCO3-) ion concentrations dissolved in the water. It is represented by the letters kH. As with calcium and magnesium, these compounds are also found in town, artesian (bore) and bottled water, and as such, their concentration is dependent on the source of the water and the treatment processes it has undergone.
Carbonate hardness helps stabilize the pH value and prevents dangerous drops in the pH value (acid drop or pH crash). It is sometimes referred to as alkalinity. An aquarium with a low kH level (50 ppm or less) will tend to be acidic. Aquariums with these characteristics are subject to rapid shifts in pH, if not monitored carefully. Water with a high kH level (> 200 ppm) usually has a high pH. The biological breakdown processes in an aquarium or pond, and the carbon dioxide consumption by plants, deplete the carbonate hardness resource in the water. Drastic changes in pH values can harm fish.
The term temporary hardness is given in recognition of the fact that bicarbonate hardness can be reduced or eliminated by boiling. This results in the formation of insoluble carbonates which are then deposited as scale or ‘fur’. General or permanent hardness cannot be reduced or eliminated in the same manner.
How to Determine the ºdH
In order to determine the hardness of your tap water, it is possible to consult the local water authority which is legally obliged to provide details of their analysis. However, it is important to realize that these values fluctuate from time to time, especially after rainfall in the region. Local fish stores offer very easy, relatively inexpensive colorimetric tests to gauge the hardness of your tap and aquarium water with reasonable accuracy.
How to Soften Water which is too Hard
The ideal solution is to collect rain water and dilute it with tap water until the desired hardness is obtained. Unfortunately, this method requires various precautions and involves certain risks. The guttering and storage tank must be of non-metallic material. They must be as clean as possible, and cemented roofs must be avoided (essentially cement is powdered limestone). This activity cannot be carried out in regions where the atmosphere is highly polluted because the rain will contain all sorts of unsuitable elements. Water collected close to or downwind of, industrial areas may be polluted. Collect only during prolonged downpours, and wait a few minutes while the dust and any other rubbish is washed away. It is highly advised to filter or strain the collected water through filter wool to remove any detritus.
Boiling water will remove some, but not all, of the dissolved salts.
You can also pass the water through resins, which capture the mineral salts and leave it very soft. This method, which is known as ion exchange, requires a well regulated flow of water and the resins to be changed regularly (use only resins sold for aquarium use). This may affect the pH, and, as it exchanges calcium ions for (usually) sodium ions, the result may be soft but still mineral-rich (and unsuitable for fish from mineral-poor regions).
In recent years a technique called reverse osmosis has been made available to the hobbyist. This involves utilizing a unit to produce water which is chemically pure, hence perfect for aquarium use. This removes all minerals but it is wasteful – some 45.5 litres of tap water are needed to produce 4.5 litres of mineral-free water. It cannot be used alone because when devoid of all its salts, the water becomes very unstable. The pH can then fluctuate widely in a short space of time, becoming dangerous for the fish. Also, the process involved in the removal of minerals, removes free oxygen, so there is nothing to “breathe”. It must be aerated heavily before use, and never used “neat”. However, the greatest drawback of this process lies in its high cost.
Filtering water through peat can effectively reduce the hardness of the aquarium. Products such as sera super peat (sera biopeat granulate in ponds) or the addition of sera morena, which contain natural peat extracts, trace elements and humic acids, can help to lower levels for softer water species. Other sera products such as pH-minus will also help to reduce hardness, in conjunction with water changes.
Finally, it is also possible to use a very soft bottled mineral water, either for dilution or for the small tanks used to breed fish. This water needs to be vigorously aerated because it is void of oxygen.
How to Harden Water which is too Soft
The simplest way to increase the general hardness is to incorporate calcareous material (e.g. limestone, crushed marble, lime sand) into the décor or filter. This will slowly release calcium carbonate into the water. Carbonate hardness can also be raised through the gradual addition of sodium bicarbonate.
Pool salt, non-iodised salt Rift Lake Salt additives (e.g. Seachem products, sera gH - kH plus, sera mineral salt etc.) and some bottled hard mineral water (e.g. Evian) can also be added. Some aquarists even add a small percentage of sea water to the aquarium (around 2%).
Proprietary treatments for raising the level of hardness, in the form of powders (e.g. kH Generator), are also available through the local fish shops.