# Does Mg lower KH?



## HiNtZ (28 Jun 2018)

As title really - when dosed, does it have an effect on the KH?

Cheers


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## Edvet (28 Jun 2018)

https://en.wikipedia.org/wiki/Carbonate_hardness
so no
http://www.theaquariumwiki.com/wiki/General_Hardness
so yes


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## HiNtZ (29 Jun 2018)

Lovely, thanks. I had a reefer friend mention something about Mg having a relationship with KH in marine environments and it got me thinking.


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## ceg4048 (29 Jun 2018)

HiNtZ said:


> Lovely, thanks. I had a reefer friend mention something about Mg having a relationship with KH in marine environments and it got me thinking.


The reefer is wrong in his words but is right in his concept.

This is yet another example of poor choice of words causing confusion which leads to misconception.
Folks on this board bash me for correcting their words (which they consider insulting) but the reason I persevere is specifically to make these concepts clear. Also this is another example of how test kits ruin the world.

The moniker KH is meant to represent the expression "Carbonate Hardness", which is a measure of the the concentration of Carbonate ions (CO3) as well as Bicarbonate ions (HCO3). Generally, from our perspective, the effects of these ions is to act as a buffer to reduce the effect of acids in the water. This is the reason Bicarbonate is taken orally by us when we overeat, which causes a rise in stomach acids.

There are however, other substances in the water that can buffer the water against acids.
These ions specifically include:
Nitrate (NO3), Potassium (K), Sulfate (SO4), Chloride (Cl), Sodium (Na), Calcium (Ca) and... Magnesium (Mg).

These ions are not as effective as CO3 or HCO3 at buffering the water against acid but they do have an effect. This effect of buffering water against pH changes due to acid is called Alkalinity.

The "Total Alkalinity" of the water includes the buffering power of Carbonate/Bicarbonate as well as the buffering power of the other ions listed above.

When CO2 reacts with water containing CO3/HCO3 there is some interaction. This interaction is shown on the ubiquitous pH/KH/CO2 charts.

Hobbyists continue to misdiagnose their problems because they measure KH with a KH test kit. The kit is incapable of isolating Carbonate Harndess from Total Alkalinity. If there is a significant presence of the other ions in the water the test kit returns a value the hobbyist assumes is KH - which it is NOT.
They plug that number into the chart and the chart returns a high CO2 value, so the hobbyists assume their CO2 concentration is high and they chase phantom causes for their plant health problems, when all the time, their CO2 is actually low and their plants suffer CO2 starvation.

We must pay attention and remember that KH test kits do not measure KH by default. If your water supply has high concentration of CO3/HCO3 relative to these other ions then the number returned from the test kit may be in fact very close to actual KH, but if these other ions are present in significant amounts then the number returned from your KH test kit is NOT KH. It is Alkalinity and the KH portion of that Total Alkalinity CANNOT be determined.

So when HiNtZ's reefer friend mentions something about Mg having a relationship with KH what the friend really means is that Mg is having a relationship with Alkalinity. The same relationship is true in freshwater.

Do NOT use the pH/KH/CO2 chart to determine the CO2 concentration of your water. The number will almost ALWAYS be a false high.
Use a 1 pH drop from gas on to lights on as a general guideline for CO2 injection. Monitor the health of your fish while you apply the injection rate to drop the pH. This will, more or less, provide a nominal CO2 concentration.

Cheers,


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## HiNtZ (3 Jul 2018)

ceg4048 said:


> The reefer is wrong in his words but is right in his concept.
> 
> This is yet another example of poor choice of words causing confusion which leads to misconception.
> Folks on this board bash me for correcting their words (which they consider insulting) but the reason I persevere is specifically to make these concepts clear. Also this is another example of how test kits ruin the world.
> ...




I'm a stickler for correct terminology in areas where I am accomplished (mechanics, engineering, etc) so I don't take offence when someone corrects me in an area where I'm not.

So if I understand correctly: By adding Mg (or any of the other elements) it doesn't have an effect on the carbonate hardness (the real KH) value - but it contributes to the total alkalinity alongside the carbonate hardness?

Cheers Ceg


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