General Hardness and Carbonate Hardness
Here's a
lengthy good read about it from
http
/www.drhelm.com/aquarium/chemistry.html
Water hardness
Water accumulates many dissolved substances before it reaches our taps. Hardness is a measurement of the concentration of metal ions such as calcium, magnesium, iron, and zinc. Most of these concentrations are acquired as rain water passes over rocks. In most water it consist mainly of calcium and magnesium salts, with trace amounts of other metals.
There are two types of hardness that we need to consider. Permanent hardness and alkalinity (often referred to as carbonate or temporary hardness) (kH). The sum of both types of hardness is called general hardness (gH)
Alkalinity or temporary (carbonate) hardness (kH) refers to the hardness derived mainly from carbonate and bicarbonate ions and directly reflects the buffering capacity of the water. It can be precipitated and removed by boiling the water. This is why lime-scale forms in kettles and shower heads.
Permanent hardness measures ions such as nitrates, sulphates, and chlorides etc, and cannot be removed by boiling.
While there is a connection between water hardness and buffering, hardness is a product of mainly calcium and magnesium ions and buffering is produced by bicarbonate and carbonate ions. As mentioned earlier, hard water is usually well buffered and soft water is usually less buffered. It is possible though, based on different water compositions, to have hard water that is poorly buffered or soft water that is well buffered. The way to establish the makeup of your local water is by using a test kit and test for both gH (general hardness) and kH (temporary hardness).
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. How much buffering does your tank need? The higher the kH (the bigger the sponge), the more resistant to pH changes your water will be. A tank's kH should be high enough to prevent large pH swings over time. If your kH is below roughly 4.5 OdH, you should pay special attention to your tank's pH (e.g., testing periodically) until you get a feel for how stable the pH is.
Buffering is both good and bad. On the good side, the nitrogen cycle in our tanks produces nitric acid (nitrate). If we don’t have 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 your fish, this large buffering capacity will make it more difficult to lower the pH.
Buffering is sometimes referred to as "alkalinity" but should not be confused with "alkaline". Alkalinity refers to buffering and alkaline refers to a solution that is base rather than acid (pH).
Aquariums with a low kH will require more attention to water changes to control the nitrate level reducing the tendency for the pH to drop.
As with pH, there are ways to increase and decrease the buffering capacity of your water.
Ways to increase kH:
· Adding sodium bicarbonate (baking soda). One teaspoon of baking soda added to 50 liters of water can raise the kH of the water by approx 4 OdH without a major affect on pH.
· Adding an air stone to increase surface turbulence driving off carbon dioxide (CO2)
· Adding commercially available products to increase buffering capacity
Ways to lower kH
· Injecting carbon dioxide (CO2)
· Use reverse osmosis (RO) water. You can mix tap water with reverse osmosis water to achieve the desired kH.
· Adding commercially available products to decrease the buffering capacity.
It is not a good idea to use distilled water in your tank. By definition, distilled water has essentially no kH. That means that adding even a little bit of acid will change the pH significantly (stressing fish). Because of its instability, distilled (or any essentially pure water) is never used directly. Tap water or other salts must first be added in order to increase its gH and kH.
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.
Most test kits measure gH or general hardness in German degrees hardness or OdH, which is equal to 17.9 mg/L. Since mg/L is equal to ppm (parts per million) simply multiply the degrees OdH times 17.9 if you prefer to work with ppm. The following table will give an idea of how hard your water may be after reading the test results.
General Hardness Table
0 to 4 dH 0 to 70 ppm Very Soft
4 to 8 dH 70 to 140 ppm Soft
8 to 12 dH 140 to 210 ppm Medium Hard
12 to 18 dH 210 to 320 ppm Fairly Hard
18 to 30 dH 320 to 530 ppm Hard
Higher Very Hard
Ways to increase gH
Adding limestone to the aquarium (this will also increase kH which in turn will increase pH)
Adding calcium carbonate will raise gH and kH
Ways to reduce gH
Adding peat moss to your filter
Use commercially available water softening pillows or a water softener (this removes calcium and magnesium ions and replaces them with sodium ions. Many people feels that this is an unacceptable method of softening water as many fish that prefer soft water don’t like sodium either.
Mixing tap water with reverse osmosis (RO) water.
It is more difficult to change gH without affecting kH than it is to change kH without affect gH as you can see.
Conclusions
While distinct, pH, kH and gH interact and affect each other. If you change one parameter, be sure and monitor the others to see the affect.
It is easiest and best to raise fish that are compatible with the water parameters you are dealt.
Make changes gradually.
When making changes it is usually best to do it in containers outside the aquarium, then add the treated water to the aquarium.
If you have a low kH, increase water changes accordingly and monitor pH more frequently.
Understand that decorations such as driftwood, bogwood, limestone, filtering with peat, etc. will affect the kH and pH of the aquarium.
Sorry, it's very lengthy. It's a good read. I had to re-read it a few times when I first read it!
Though it's mbuna not Bhuna. 
Although, the fish do like eating it.)
