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Discussion Starter · #1 ·
Just wondring :D

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There are actually two types of hardness.
GH-general hardness
KH-carbonate hardness

Carbonate Hardness(KH)
This is an extent of the temporary hardness of water. This is derived mainly from carbonate and bicarbonate ions and directly reflects the buffering capacity of the water. It can be removed by boiling the water. (This is why lime-scale builds up on heater elements.)

To raise KH
1. Aerate the water, driving off the carbon dioxide (CO²).
2. Add a commercially available product to increase buffering capacity.
add baking soda.

To lower KH
1. Injecting carbon dioxide (CO²).
2. Add a commercially available product to decrease the buffering capacity
Perform water changes with RO (Reverse Osmosis) water.

General Hardness(GH)
Permanent Hardness.
This is derived mainly from the concentration of magnesium and calcium ions. (Other ions can add to water hardness but are usually irrelevant and not easy to measure.)
Note: It is GH, not KH that is being referred to when talking about fish that prefer soft or hard water.

0 to 4 OdH or 0 to 70 ppm = very soft
4 to 8 OdH or 70 to 140 ppm = soft
8 to 12 OdH or 140 to 210 ppm = medium hard
12 to 18 OdH or 210 to 320 ppm = fairly hard
18 to 30 OdH or 320 to 530 ppm = hard
above 30 OdH or 530 ppm=very hard

To raise GH
1. Add limestone to your tank. (this will also increase KH which in turn will increase pH)
2. Add calcium carbonate will raise both GH and KH.

To lower GH
1. Use a water softening product.
2. Mix tap water with RO (reverse osmosis) water.

2,716 Posts
I would never use boiled water in any fish tank... it will bottom out the pH rather quickly. I've done a lot of experimenting with this recently, and determined that boiled water is unsafe for ANY fish tank, fresh or salt water.
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