KH and GH give different CaCO3 results?

[jacksdad]

Just noticed this, I've tested my tanks, on each one of three, the GH and KH readings give completely different CaCO3 results.
Shrimp Tank: Alkalinity KH = 2° dH and 35.6 CaCO3
General Hardness GH = 3° dH and 53.4 CaCO3

160 litre tank: Alkalinity KH = 3° dH 53.4, CaCO3
General Hardness GH = 6° dH, 106.8 CaCO3
I've gone back and checked and every test I've done the KH and GH results are different! I've only just checked the conversions to CaCO3, how can they be so different?

[Stephen]

CaCO3 = calcium carbonate
53.4 = ppm (parts per million)
GH = General hardness
KH = Carbonate hardness

GH & KH are two different things.
35.6ppm = approx. 2dGH (degrees general hardness) or 2dKH (degrees carbonate hardness) depending on what is being measured.
53.4ppm = approx. 3dGH (degrees general hardness) or 3dKH (degrees carbonate hardness)
106.8ppm = approx. 6dGH (degrees general hardness)
All use the German measure of hardness whether GH or KH.

It is likely that something in the 160L tank is pushing up the hardness and making it slightly hard. This could be something such as gravel or rocks or even a plant fertiliser (or something else).

[jacksdad]

I know all that: The 160 is raised by the remains of gravel I initially used, my point is that when I convert the KH and GH/dH results to CaCO3, they are different.
The 160 is the worse, when I do the KH test, the CaCO3 is half the number given in the GH test.
I haven't given the 260l tank results, I'm doing a water change tomorrow so I'll test it again and see how far apart the (KH and GH) CaCO3 results are, I know they will be different.

[Gingerlove05]

It makes sense to me that they would be different.
Although normally if you have a high gh you will have a high kh and vice versa, the gh is the measure of magnesium and calcium where as the kh is a measure of carbonates in your water.
The calcium and magnesium are what make your water hard/soft depending on levels, whereas the carbonates buffer your ph and help keep the ph stable. So the tests measure different parts of water hardness

[Stephen]

GH & KH are different things and not really related. GH measures the calcium & magnesium plus a few other minerals whilst KH measures the carbonates & bicarbonates.
KH is often associated with pH, the higher the KH then usually the pH is higher, it is the KH (carbonates & bicarbonates) that buffer the pH as GL states above.
The old gravel remains are likely releasing minerals that has pushed the GH up and likely releasing carbonates & bicarbonates which has pushed the KH up.
The fact that your KH is half that of the GH in your 160L tank is not relevant as both are different things.

[jacksdad]

I do understand that KH and GH are different, honest!
I know the 160 has higher GH because of the gravel: in my other posts I gave the results of a test of the gravel in a small tub of gravel/water, it went to double the tap water Gh in a week, so I removed most and replaced with inert Limpopo sand, but the remains still bump up the level a little. Its not an issue as the swordtails are in that tank and breeding well.
Can you answer this....what is the CaCO3 level in my 160? What is the dH number?

"160 litre tank: Alkalinity KH = 3° dH 53.4, CaCO3
General Hardness GH = 6° dH, 106.8 CaCO3"

[Gingerlove05]

In the KH test its degrees hardness but only carbonate hardness so it should be dKH, the GH test is degrees hardness and i think its german hardness as thats most commonly used, so that should be dGH.
Hope that makes sense

[Mol_PMB]

When salts are dissolved in water, the ions separate. So in your tank you have Ca ions and CO3 ions, separately.

If you were to add some potassium carbonate, you would have K ions and CO3 ions.

If you were to add some Calcium chloride, you would have Ca ions and Cl ions.

The GH test measures the concentration of Ca and Mg ions.

The KH test measures the concentration of CO3 ions.

They do not have to be the same.

[jacksdad]

Still not sure what my test results show, regarding CaCO3. Are you saying the CaCO3 in the Alkalinity KH test, is different to the CaCO3 in the General Hardness test?
So, if I was to say what my CaCO3 level is, do I say 53.4 or 106.8?

[Mol_PMB]

There is no such thing as CaCO3 in the water. There is Ca and there is CO3.

The KH test measures the amount of CO3, while the GH test measures the amount of Ca (and Mg).

If you took pure RO/DI water and added sodium carbonate, you would get a CO3 reading (KH test) but zero on the GH.
If you took a different sample of pure water and added calcium chloride, you would get a Ca reading (GH test) but zero on the KH.

Because GH measures both Ca and Mg, some plant fertilisers such as Magnesium Sulphate can contribute to a raised GH reading without affecting the KH.