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Discussion Starter · #1 ·
I'm new to CO2 (only doing 24x7 DIY right now) and I've only recently dove into understanding the relationship between CO2, KH, and PH. I started testing my tank this week for PH/KH, and now I'm more confused and I'm trying to figure out what's going on.

Directly from the faucet, tested immediately, my tap water is:

PH 7.4
KH 18

According to the KH/PH charts, I have about 20ppm CO2 in my tap. This is confirmed by letting my tap water sit in a cup for a day, and PH goes to 8.2.

I did a 50% water change last Sunday, as I do every Sunday since I am dosing EI. I did another 50% water change on Wednesday because my nitrates were running a bit high, and out of curiosity I took a cup of tank water just before the change and let it sit out. I finally tested it today and I got:

PH 8.2 (expected)
KH 10 (uhh... huh?)

KH at 10? What? Where did all that KH go? I was expecting that if I kept putting 18 KH water in, I should get 18 KH water out. Since it was still morning and my lights had not come on yet and my drop checker was still yellowish, I took a quick tank sample and I got:

PH 6.8 (37ppm CO2 according to the charts)
KH 8

So my tank was at KH 10 on wednesday, and I swapped 1/2 of the water with water that has a KH of 18, and now just 3 days later KH is at 8? I tested it tonight just before lights out and I got:

PH 7.0 (around 21ppm CO2 according to the charts)
KH 7

Tomorrow will be another 50% water change, but I'm curious why my KH drops so much. Here are my tank parameters (up and running for 6 weeks):

12 Gallon Fluval Edge (17 x 10 x 18)
27W PC Light 16" above substrate
DIY CO2
ADA Aqua Soil (a little less than a 9L bag)
Relatively heavily planted for it's size
Dosing at 1/2 EI

Is Aqua Soil lowering my KH that much? Or is it because I'm injecting CO2? Does injecting CO2 and lowering PH cause some reaction where KH tries to raise PH back up and get's used up or something? Or does something occur in the nitrogen cycle that eats up KH? Or is it something completely different?

Mind you my tank is fine, my plants are growing nicely, and my fish/shrimp are happy, so there is no "problem" per se — it's just a scientific curiosity.
 

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Hi there,

Let me share some of my findings on this problem as I had a hard time understanding this myself. There's a TON of misinformation on the topic.

First of all, the pH-kH-CO2 table is pretty much useless in giving you accurate information. The only value you can find out of the table is pH. Given you know exactly how much CO2 you have in the water.

When you understand how CO2 changes pH you will also find out why the table is useless :)

Also, if you have anything in the water that alters kH or pH that is not CO2 it will mess with that table as well, making it even more useless.

Let's start with a bit of chemistry here:

The reactions that occur when CO2 dissolves in water are somewhat tricky to grasp as they cannot be measured exactly even in lab conditions. They are mathematically calculated based on laws established by chemists.

As water is neither a base or an acid it makes hard to pinpoint exact numbers as the numbers themselves change as the reaction progresses.

Let's get down to business with the reaction of CO2 with water:

Code:
+++       <=>>      <=>>     ---   <=>>
CO2 + H2O <=> H2CO3 <=> H+ + HCO3- <=> CO3^2- + 2H+
H2CO3 is a weak acid, in water it should quickly dissociate to H+ + HCO3- making the pH go down. If we add more bicarbonates into the water it will shift the equilibrium H2CO3 <<=> H+ + HCO3- thus buffering the pH.

Code:
+++       <=>>     <<=>      +++   <=>
CO2 + H2O <=> H2CO3 <=> H+ + HCO3- <=> CO3^2- + 2H+
Lack of bicarbonates will shift the equilibrium H+ + HCO3- <=>> CO3 + 2H+ plummeting the pH.

Above we have:

H2CO3 = carbonic acid
HCO3- = bicarbonate
CO3^2 = carbonate

This is a gross explanation of what reactions are taking place as I'm no chemist. I asked around myself and read quite a lot about some chemistry reactions before getting to this conclusion on how kH buffers the pH and how CO2 lowers pH.

If the above reaction doesn't make sense to you, read up about pH and what it means over on Wikipedia and then look at the Hydrions in the above reactions ;)

Hope this helps, if there is someone here which is more knowledgeable with chemistry, I could use some corrections if I said something stupid :p
 

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Discussion Starter · #5 ·
Yeah, it's gotta be the soil. I'm just surprised it's lowered KH that much. Today before my water change the tank was at 6. After the water change it's predictably at 12 since my tap is 18. It looks like it lowers mine by about 1 KH per day.

How long does Aqua Soil have this effect?
 

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Pretty much forever. I wouldn't worry about it. Again, KH is a pretty meaningless parameter for fw aquarists.
 

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Yeah, it's gotta be the soil. I'm just surprised it's lowered KH that much. Today before my water change the tank was at 6. After the water change it's predictably at 12 since my tap is 18. It looks like it lowers mine by about 1 KH per day.

How long does Aqua Soil have this effect?
A long time, in years maybe. As the tank matures, more plant decomposing matter will leach fulvic and humic acids just like the soil does carrying some of the soil's properties on as it depletes.

AS does what pretty much peat does with regards to kH but it's a mix of multiple ingredients.

Also, there are some plants that can readily use carbonates and bicarbonates as their source of carbon although all of them prefer CO2 over anything else.

Just like the case with glut. Plants do not have a taste for glut and will use it if there is no other source of carbon for which they are designed to use: CO2.

On the upside, lower kH means more CO2 is available to plants. Raise the kH and you will notice plants stop pearling, lower it and they will resume pearling.
 

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Discussion Starter · #9 ·
I wouldn't worry about it. Again, KH is a pretty meaningless parameter for fw aquarists.
plant like lower PH/KH so dont worry about it.
Yeah, like I said in the original post, it's not a problem at all and I wasn't worried, just a scientific curiosity. It's a big plus though, because I had purchased some rummy nosed tetras before I knew they were soft water fish, and this is definitely helping them out. :proud:
 

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Pretty much forever. I wouldn't worry about it. Again, KH is a pretty meaningless parameter for fw aquarists.
It will do this until it runs out of buffer..
It cannot repeatedly buffer water. Especially harder water like that forever
I give it about a year. But im no AS expert, i just know that its buffering capacity is finite. Ask any serious shrimper, they of all people appreciate and understand buffering substrates
 
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