By Donna M. Recktenwalt
Maintaining fish in an artificial environment poses a number of questions to the aquarist, but one thing we all must deal with is water.
We know that for our fishes to thrive, their water must be clean, free of chemical contaminants, and capable of retaining dissolved oxygen. It also must fall within the appropriate temperature, pH, and hardness parameters.
For a simple item, then, water poses complex questions.
In this article, we will discuss the matter of hardness.
Water may be “soft” (very little dissolved mineral content), or “hard” (a great deal of dissolved mineral content). In the home, the greatest differences noticed between the two may be the sudsing levels of soaps and the amount of “scale” buildup in pipes. For the aquarist, the level of hardness has a direct effect on the pH buffering ability of the water. Some of us may draw our water supply from a municipal source, some from local wells, some from collected rainwater. Some aquarists have water so hard – or so dissimilar to that required by the fish they keep – that they have installed Reverse Osmosis systems to assure a steady supply of suitable water.
Total Water Hardness, or General Hardness (GH), which is measured by most water test kits, consists ofTemporary Hardness plus Permament Hardness.
Temporary Hardness, also known as Carbonate Hardness (KH) is a measure of the bicarbonates precipitated upon boiling, and consists mostly of Calcium and Magnesium salts. This is the “scale” that accumulates inside a teakettle or pot. It is Carbonate Hardness (KH) which determines the pH buffering capacity of water
Permanent Hardness refers to sulphate and chloride salts, which do not precipitate upon boiling.
To confuse the issue farther, there are at least four different scales of hardness in general use: US, Clark (UK), German (dH), and French (fH). The table which accompanies this article offers a comparison among them. All values are calculated in equivalent CaCo3 in milligrams per liter.
In most cases, published species information includes water hardness and pH preferences. However, most species are adaptable. For new species that have not yet made their way into the literature, determining appropriate conditions may prove a challenge. However, the following may provide some guidance.
In most cases, South American annuals prefer softer water and Nothobranchius species hard water. Barry Cooper indicates that he keeps his South Americans in soft water (100 ppm TDS), with or without the addition of salt. Most SA annuals, he says, don’t seem to particularly mind acid water. Nothobranchius species, on the other hand, he maintains in water that measures TDS 250-280, plus 1/2 tsp. salt per gallon. In the wild, Barry notes, most Nothos are found in alkaline water.
Jorgen Scheel recommends hard, alkaline water with a pH of up to 8.0 for killifish.
Many breeders have found that Aphyosemion species do well in half tap water (hard) and half soft water.
Comparison of US, British, German, and French Hardness Designations (mg/L)
Term / US (1) / Clark (UK) (14.3) / German (dH)(17.9) / French (fH) (20)
Soft /US 0-50 / Clark 0-3.5 / German 0-2.8 / French 0-2.5
Moderately Soft / US 50-100 / Clark 3.5-7. 0 / German 2.8-5.6 / French 2.5-5.0
Slightly Hard / US 100-200 / Clark 7.0-14. 0 /German 5.6-11.2 / French 5.0-10.0
Moderately Hard / US 200-300 / Clark 14.0-21. 0 / German 11.2-16.8 / French 10.0-15.0
Hard / US 300-450 /Clark 21.0-31.5 / German 16.8-25.1 / French 15.0-22.5
Very Hard / US 450+ / Clark 31.5+ / German 25.1+ / French 22.6+
— G.C.K.A. Newsletter – February 1997