kH: Carbonate hardness or temporary hardness. Measures the buffering capacity or the ability to absorb and neutralize added acid without major changes to pH. Think of buffering capacity as a big sponge, the higher the buffering, the bigger the sponge. The higher the kH, the more resistant to pH changes water will be. A tank's kH should be high enough to prevent large pH swings over time. If kH is below roughly 4.5 OdH, special attention should be paid to the pH. Buffering is both good and bad. On the good side, the nitrogen cycle in our tanks produces nitric acid (nitrate). If there is no buffering (kH), the pH will drop over time. Sufficient buffering will keep the Ph stable. On the bad side, hard water almost always has a large buffering capacity and if the pH is to high for fish, this large buffering capacity will make it more difficult to lower the pH.
gH: General hardness (GH) refers to the dissolved concentration primarily of magnesium and calcium ions. Other ions can contribute to water hardness but are usually insignificant and difficult to measure. When fish are said to prefer ``soft'' or ``hard'' water, it is gH, not kH that is being referred to. gH will not directly affect pH although "hard" water is generally alkaline due to some interaction of gH and kH. 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.
GENERAL HARDNESS TABLE
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 Very Hard
