Understanding General And Carbonate Hardness


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I am sorry to post but I am at a loss after hours of looking online and still not understanding the information of General Hardness and Carbonate Hardness.

My partner ordered this test kit for parameter checks to house freshwater fish in one of our tanks and I do not understand the readings and what General and Carbonate Hardness all means in the use of plants and certain species of fish.

My Parameters are different for all 4 tanks I have and are as follows
3.5-gallon tank GH=143.2ppm/KH=89.5ppm
10-gallon tank GH=268.5ppm/KH=107.4ppm
30-gallon tank GH=143.2ppm/KH=125.3ppm
40-gallon tank GH=214.8ppm/KH=89.5ppm

I can give the specifics of each tank if asked as it is a lot to list all of it.

What I am trying to gain is a better understanding of what GH and KH mean and how it relates to a planted tank community.


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3 years
General hardness (GH) is primarily composed of calcium, magnesium, and potassium.
Carbonate hardness (KH) is composed of mostly calcium carbonate (CaCO3) which is different from calcium.

General hardness does not affect ph much, if at all.
Carbonate hardness buffers ph, causing it to stay at a certain point. The higher the KH, the higher the ph, and the harder it is to alter the ph.

Certain fish that come from soft water streams and lakes prefer low GHs, especially for breeding and for displaying colors.
They may also prefer lower KH.

Usually when speaking about hardness, we use the german measurement of degrees rather than ppm.
1 degree = 17.8 ppm.

In my own opinion:
0-2 degrees = very soft water
3-4 degrees = soft water
5-8 degrees = moderate water
9-12 degrees = hard water
12+ degrees = very hard water

Plants require calcium, magnesium, and potassium in GH, because they are important macronutrients. Macronutrients are the nutrients plants require more of, and micronutrients are the ones they need smaller amounts of. People like to talk about dosing iron a lot, but iron is actually a micronutrient and is needed in lesser amounts.

The lowest ideal GH level for a planted tank is around 4-5 in my opinion, though 6 or more may be better if your fish can tolerate it.
KH does not matter to plants.

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