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What is water hardness?

Written in response to the question "What is the difference between General hardness and total hardness and how does it relate to KH?"

My basic understanding is this; you have various ions that cause "hardness". I think that what causes this in water is the carbonate ion such as is found in calcium carbonate, magnesium carbonate etc.

When you have an ion exchange resin or pillow, all it will do is swap them for sodium carbonate, so that the hardness is no longer detectable by the same tests, but there are still carbonates found in water in a different form, so it's not any softer in reality. This is why people often say don't bother buying softener pillows but if you REALLY want to soften water use another way like filtering over peat, or just live with the water you get out of the tap.

Originally, hardness was a measure of the ability of water to precipitate soap, and this is affected by calcium and magnesium compounds. Therefore, total hardness - and also "general" hardness - is a measure of all the calcium and magnesium salts in the water, such as (but not limited to) calcium carbonate and magnesium carbonate.

Carbonate hardness (KH) is a measure of all the carbonate compounds in the water. It is important, because this acts as a buffer for pH - the higher the hardness, the more difficult it is to alter the pH, so if you have a KH of 2 or 3 it's quite easy for a tiny change to cause your pH to crash. If your KH is say 13 or 14 you will find your pH varies very little, no matter what you do!

For what it's worth, I've tried using pH down and pH up (with little success because I had a large KH buffer) and then tried using a powder that you tip in to lower pH - again no good, it formed a cloudy suspension in my hard water and it was ages before the tank was clear again, by which time the pH was back to normal - and now the only way I reduce my pH is by filtering over peat and then mixing that water with tap water to get the right pH and KH, then adding that water to the tank.

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