Gh/kh and C02 levels - The Planted Tank Forum
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post #1 of 6 (permalink) Old 01-24-2017, 06:22 PM Thread Starter
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Gh/kh and C02 levels

So I'm wondering how this could be affecting my tank. I have a a GLA c02 system on a timer. I currently do not have a drop checker but I turned the c02 down a couple notches when I noticed my puffers gasping at the surface.
I try to keep my gh 4 and kh 4. I accidentally boosted the kh to 5 on Sundays water change. Because of the snail shells that the puffers leave to dissolve, my gh goes up to 7 by the time my next Sunday. I did a 50% waterchange on sunday as usual because of EI. But it only brought the gh down to 5 this time. I haven't had to add any equilibrium for 4 weeks so far. Water from my tap is 0gh 0kh. How do these fluctuating gh and kh affect my C02 concentration? And by how much? And since I'm not adding any hardness to the water won't the gh reading eventually go down? I mean I am removing hardness every time I do a water change, they gotta start running out of minerals eventually. I would think the gh increase would start slowing down.
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post #2 of 6 (permalink) Old 01-24-2017, 06:27 PM
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Hi LittlePinkDot,

I see you are on the 'Wet Coast' as it is called up there!

The dKH and dGH do not effect your CO2 concentration at all. When you do a water change that will typically reduce the CO2 concentration for several hours until it has an opportunity to build back up.

PS: Tom Barr is talking on Seattle April 14th, why not come to Seattle for Valentine's Day, visit GSAS (as a guest) and hear the talk!

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post #3 of 6 (permalink) Old 01-24-2017, 11:42 PM Thread Starter
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Originally Posted by Seattle_Aquarist View Post
Hi LittlePinkDot,

I see you are on the 'Wet Coast' as it is called up there!

The dKH and dGH do not effect your CO2 concentration at all. When you do a water change that will typically reduce the CO2 concentration for several hours until it has an opportunity to build back up.

PS: Tom Barr is talking on Seattle April 14th, why not come to Seattle for Valentine's Day, visit GSAS (as a guest) and hear the talk!
Can't make it there. I don't have a drivers license.
If kh does not affect c02 levels then why is kh on that c02 chart along with ph?
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post #4 of 6 (permalink) Old 01-25-2017, 02:08 AM
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Can't make it there. I don't have a drivers license.
If kh does not affect c02 levels then why is kh on that c02 chart along with ph?
I think what Roy is saying is that raising your KH won't measurably decrease or increase CO2. KH buffers acids such as carbonic acid, so you have to inject more CO2 at higher KH to achieve the same pH drop as lower injection rate at lower KH.

That chart is definitely not perfect, and I think it is commonly misused. You want to look at what your pH and KH normally are without CO2 injection, then look at the rough ppm of CO2 you want and figure out how much of a pH drop is required to reach the ppm you desire. Higher KH will just require more CO2 injection to reach the desired pH drop (and ppm).
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post #5 of 6 (permalink) Old 01-25-2017, 03:20 AM
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If you want 20 ppm of CO2 in your water you have to diffuse 20 ppm of CO2 into the water, and keep it up at a rate that replaces the CO2 used by the plants, and lost from the water surface to the air. It makes no difference how much GH or KH you have.

If you want to try to measure how much CO2 is in the water you can measure the KH and the pH of the water and those charts will give you a number which will most likely be much higher than the amount of CO2 you actually have in the water. This is because the charts don't work at all accurately with typical aquarium water.

Raising the KH will also raise the pH, even if you only have the 3 ppm that we typically get from the CO2 in the room air. Changing the GH will do nothing except raise the GH.

Buffering is done by a mix of a weak acid and the conjugate base of that acid - meaning the chemical formula for the conjugate base is the same as that for the acid, but minus one hydrogen atom, making it an ion in the water. If you mix CO2 in water a small portion of the CO2 forms carbonic acid in the water, and the conjugate base for carbonic acid is the carbonate ion. So, a mix of a carbonate, like sodium bicarbonate, which becomes a mix of carbonate ions, plus some anions, and carbonic acid, which forms from CO2 in the water, is a buffer. That buffer holds the pH constant at a value that depends on the amount of carbonates in the water (KH) and the amount of dissolved CO2 in the water. It buffers the water against changes in pH that would be caused by other weak acids or bases in the water.
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post #6 of 6 (permalink) Old 01-25-2017, 04:41 AM Thread Starter
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Quote:
Originally Posted by LittlePinkDot View Post
Can't make it there. I don't have a drivers license.
If kh does not affect c02 levels then why is kh on that c02 chart along with ph?
I think what Roy is saying is that raising your KH won't measurably decrease or increase CO2. KH buffers acids such as carbonic acid, so you have to inject more CO2 at higher KH to achieve the same pH drop as lower injection rate at lower KH.

That chart is definitely not perfect, and I think it is commonly misused. You want to look at what your pH and KH normally are without CO2 injection, then look at the rough ppm of CO2 you want and figure out how much of a pH drop is required to reach the ppm you desire. Higher KH will just require more CO2 injection to reach the desired pH drop (and ppm).
Hmmm my water is 7.0 ph from the tap. I add the kh directly into the tank. I use seachem aquavitro liquid potassium carbonate. When my lights and c02 have been on for awhile, my ph is 6.4
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