Water Hardness

for the week of 11/5/98

Isn’t Hardness the same thing as pH? No. While pH is a measure of acidity, hardness is a measure of how much calcium (and to a much lesser extent, magnesium) is in the water. On the other hand, it is true that hard water is naturally likely to also have a high pH, because it is also high in carbonates that buffer the pH toward the alkaline side. (This is sometimes called “Carbonate Hardness,” but that’s another issue for another time.)

Is there an optimum level of hardness? As in the previous discussion of Optimum pH, fish come from a wide range of hardness levels, but most common fish adjust to different levels if the acclimation process is gradual. The best bet is to ask if your local dealer has a similar hardness reading. If the fish you intend to purchase appear acclimated in his tanks, they should make the change to yours. Hardness can be important in keeping certain more delicate fish (e.g. wild-caught neon tetras, Corydoras cats or Ram cichlids) and in breeding others (certain tetras, discus, and a few other cichlids).

How is hardness measured? There are two units commonly used in measuring hardness: German Degrees of Hardness (dH) and Parts Per Million (ppm). One dH = 17 ppm. For a rule of thumb:

0 to 4 dH 0 to 70 ppm very soft
4 to 8 dH 70 to 125 ppm soft
8 to 12 dH 125 to 200 ppm medium hard
12 to 18 dH 200 to 300 ppm hard
18 to 30 dH 300 to 500 ppm very hard
over 30 dH over 500 ppm extremely hard

How can water be “softened?” If water is too hard for a particular use, it can be softened by commercial “water softeners,” which remove calcium ions from the water and replace them with sodium ions. Or it can be run through a Reverse Osmosis or De-ionizing unit, both of which remove almost all dissolved chemicals from the water, including calcium. Any of these processes can produce water with a hardness of zero, which is unsuitable for fishkeeping in general. However, mixing the proper proportion of this zero-soft water with hard water yields the desired hardness – for example, adding one gallon of 0 dH to one gallon of 16 dH results in two gallons of 8 dH.

Water can also be softened, and simultaneously acidified, by filtering through a layer of peat moss, and some aquarists run such a filter directly on their aquarium. This should of course be monitored regularly to avoid drastic changes.

How can hardness be increased? A few types of African cichlids appear to appreciate harder water than some tap supplies. Adding crushed shells, limestone or marble chips, or crushed coral gravel to the filter system will increase the hardness and pH.

“Tip of the week” appeared regularly in 1999 and 2000.