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do i understand this?


Lemon
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Correct, from what I understand. The reason why people often recommend low KH for soft water fish is because they often also like low pH, and low KH makes it easier to reach low PH. 

Here's a high level blog article from Aquarium Co-Op if you're interested in water chemistry: https://www.aquariumcoop.com/blogs/aquarium/ph-gh-kh

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KH is how many carbonates you have dissolved in your water (Carbonate is CO3, bicarbonate is HCO3).  This often comes from things like crushed coral, or limestone as part of your substrate, or because the water you are using comes into contact with limestone.  

GH is general hardness, and is usually looking at the Ca and Mg you have dissolved in your water.  

The stuff that makes up your kH is a part of the carbonate buffer system.  http://butane.chem.uiuc.edu/pshapley/environmental/l24/3.html. Basically, it is a system of dissolved ions in water that favors a slightly above neutral point, and it will grab onto and release hydrogen ions into water to keep it in that slightly alkaline/basic range.  It's also very common in a lot of natural water systems.

Less of the carbonate buffers in acidic water is one of the reasons if you aren't careful, it's easy to let your pH drop precipitously into deadly territory.  You don't have ions floating around that are keeping the excess hydrogen under control.  With the carbonate buffer system, it tends to be a lot more stable, and the amount of H+ floating around in your water is more stable.  

GH and KH are often related, but not always.  You can get excess KH if you inject CO2, because the CO2 will do stuff and become part of that carbonate buffer.  Putting something like calcium chloride in your water would give you GH, but not effect your KH.

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