Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Usually the 2 are independent.
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In nature there are rocks and soils that were derived from shells, corals and similar materials. They may have many names, including limestone, dolomite, calcite and others.
In general they are high in calcium carbonate and magnesium carbonate. Often they have other minerals in them. The % of each component varies.
These rocks and soils decompose easily when they get wet. Rain especially because it is slightly acidic when it picks up CO2 from the air, and streams or rivers high in organic acids. Acidic water attacks these rocks and soils, and some of the minerals break down and enter the water.
The Ca and Mg are measured as GH.
The carbonate and bicarbonate are measured as KH.
TDS is a test of Total Dissolved Solids.
Since both GH and KH are usually present in the parent rock, both usually show up in the water that filters through these rocks.
Not always. Sometimes there are rocks that have a much higher % of one item, or the material dissolves at different rates.
However, most of the time you can group waters into any of several very broad groups:
Soft water has almost no minerals. Very low TDS. The water flows over harder rock that does not dissolve, or flows through rain forests only touching fallen leaves, branches and other organic materials. GH and KH are both low, and the pH is usually affected by the organic matter, and will be on the acidic side of neutral.
Hard water has lots of minerals. The water flows through areas with low rainfall, more exposed soils and rocks, less organic matter, and these soils and rocks can dissolve in the water. The original homes of many live bearers are like this. In another example, the Rift Lakes of Africa, the water ends up in lakes that start evaporating. This leaves the minerals behind, so the lake water keeps getting harder. GH and KH are both high, and the high KH acts as a buffer that keeps the pH in the high 7s to low 8s. TDS is also high. Many dissolved minerals in such waters.
There are certainly waters in between these two, but on average the GH and KH will usually be at least somewhat similar, though not always exactly matching. The carbonates are one of the most common buffers that control the pH, so pH may follow the KH. High KH (almost always)= high pH. low KH allows the pH to vary, and often the pH will be low when the KH is low. These are all very rough relationships, and there are other things that can alter the pH.
For some places to see more about this material google info about Karst, limestone caves (in the mountains of Kentucky and nearby states, as well as the South West). Texas Holey Rock is one material that is commonly used in aquariums where you want the GH, KH and pH to be high.
When this water is the source for a city the water company may try to do something with the water to make it more acceptable to the customers. The high mineral content makes the water difficult to wash with, and leaves hard water stains and even crystals of the minerals around sinks. What the water company does may alter the amount of Ca, Mg and KH in the water in ways that leaves the levels sort of odd.
Best method for dealing with water for fish:
Do research to make sure of what range of GH is best for the fish.
Make the GH of the water suit the fish. Make sure you are using a blend of calcium and magnesium, and do tests if needed to see if one or the other is way out of range.
Then make the KH about the same, but as much as 2 German degrees of hardness up or down is fine, and even higher or lower is not much of a problem.
When these 2 factors are right the pH will usually be at least in the ballpark. If you are dealing with black water fish I would filter the water through peat moss.