I didnt write this, I pinched it form a web site.
"GH is a measure of calcium and magnesium.
This is the hardness that you go by when you are talking about hard water fish or soft water fish. These minerals are also plant fertilizers, so if the GH is really low (like under 3 degrees) the plants might start showing signs if deficiency.
KH is a measure of carbonates. These are a buffer that stabilizes the pH. VERY roughly, high KH means high pH. Low KH means the pH will usually be low. BUT NOT ALWAYS. Other things can alter the pH that have little or nothing to do with the KH.
A KH under 3 degrees makes it easier for the pH to vary.
In natural water the KH is usually about the same value as the GH, rarely the same, but often within a few degrees higher or lower.
In nature rain water falling on rocks or soil can dissolve some of the minerals in the soil. Often the minerals include calcium carbonate and magnesium carbonate.
When there is a lot of rain, and it falls more on the trees and fallen leaves, not so much on rocks or soil, the water will be very soft. Low GH, low KH, pH neutral or acidic. Fish from tropical rain forests thrive in such water.
Where there is not so much rainfall there is not so much plant growth, and more of what rain there is lands on the soil. The rivers and lakes accumulate the minerals and the water is hard and alkaline. Fish from such lakes thrive in hard alkaline water.
Aquatic plants are very adaptable. Most of the common plants will thrive in any water that fresh water fish will thrive in, as long as the GH is not too low. pH, KH and GH do not seem to be so important to the plants. There are certainly some plants that might require more specific attention to these aspects of water chemistry, but these plants are more common among specialists. If your water is very soft, very low GH then the hard water plants are not likely to do so well. This might be why someone suggested that a KH (wrong chemical) ought to be 8 German degrees of hardness for a planted tank. It is actually calcium deficiency that causes problems to some hard water plants if the GH is too low." by DianaK
