Can someone explain how I should read this drop checker chart - The Planted Tank Forum
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post #1 of 9 (permalink) Old Yesterday, 02:50 AM Thread Starter
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Can someone explain how I should read this drop checker chart

I thought I understood this chart but one point of confusion to me is that the PPM numbers for "too little CO2" in yellow can be very high, while the PPM numbers for "too much CO2" are very low. Is this chart accurate? Is this just how the chemistry works?

I have a Seiryu rock in my tank which appears will cause some KH drift throughout the week so I am trying to see how I can use this chart to plan. But not sure how this chart is used. What numbers do you plug in? Your after CO2 pH and KH or your nominal pH and KH?

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post #2 of 9 (permalink) Old Yesterday, 05:50 AM
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My understanding is that the ppm numbers are the amount of CO2 required to drop the pH to a given level. KH resists changes in pH, so you need more CO2 to drop the pH to the same value when you have high KH. In contrast it takes very little CO2 to drop the pH in low-to-zero KH water (hence why RO water often measures quite acidic as it has absorbed a small amount of CO2 from the air).

At any KH, the optimum CO2 level is about 15-30 ppm, according to the chart. The idea is that 30ppm of CO2 will drop the pH all the way down to 6.3 if you have 2KH, but will only drop to ph7.2 if you have very hard (high KH) water.

But wait, now I'm confused! Are the yellow and red sections not the wrong way round in that chart???? How can 2ppm CO2 be "too much CO2" and 223ppm be "not enough CO2"???

Ok, give up. Let's wait for an expert to come along....


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post #3 of 9 (permalink) Old Yesterday, 06:53 AM
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Red is "Too little CO2" and Yellow regions are "Too much CO2" as per this chart.
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post #4 of 9 (permalink) Old Yesterday, 12:33 PM
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post #5 of 9 (permalink) Old Yesterday, 03:13 PM Thread Starter
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Does KH "absorb CO2"? Why wouldn't your PPM of CO2 simply be determined by your bubble count/flow rate? That's the whole confusing part of this. Since it says you have to "drop pH by 1 to get 15-30ppm".
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post #6 of 9 (permalink) Old Yesterday, 04:29 PM
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Quote:
Originally Posted by ahem View Post
I thought I understood this chart but one point of confusion to me is that the PPM numbers for "too little CO2" in yellow can be very high, while the PPM numbers for "too much CO2" are very low. Is this chart accurate? Is this just how the chemistry works?

I have a Seiryu rock in my tank which appears will cause some KH drift throughout the week so I am trying to see how I can use this chart to plan. But not sure how this chart is used. What numbers do you plug in? Your after CO2 pH and KH or your nominal pH and KH?

Attachment 903045
This chart is incorrect; the red section should be labeled "too little CO2" while the yellow section should be labeled "too much CO2"

Quote:
Originally Posted by ahem View Post
Does KH "absorb CO2"? Why wouldn't your PPM of CO2 simply be determined by your bubble count/flow rate? That's the whole confusing part of this. Since it says you have to "drop pH by 1 to get 15-30ppm".
Carbonate hardness serves as a buffering agent against changes in pH. Essentially, with a higher KH, you would have to inject more CO2 to achieve the same decrease in pH, meaning you'd be injecting more (and have a higher concentration).


The concentration of CO2 is determined by your injection rate, but you also have to take into account a multitude of other factors, such as off-gas, absorption, dissolution rate, other effects on pH such as humid acids and other dissolved organic compounds, etc, so in the end, we just simplify it to reduce the complexity.
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post #7 of 9 (permalink) Old Yesterday, 05:14 PM Thread Starter
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Originally Posted by Darkblade48 View Post
This chart is incorrect; the red section should be labeled "too little CO2" while the yellow section should be labeled "too much CO2"
Carbonate hardness serves as a buffering agent against changes in pH. Essentially, with a higher KH, you would have to inject more CO2 to achieve the same decrease in pH, meaning you'd be injecting more (and have a higher concentration).
I can't understand how that works. So animal life can handle a higher CO2 PPM because of a high KH? I have not seen that claim directly made.

That's why I was asking if KH "absorbs CO2" and thus you have to put more CO2 in to have 30PPM free dissolved CO2 with a higher KH. But from reading, only a small fraction of a percent turns into carbonic acid.

Given that the chart has this flagrant error of saying too much is too little and too little is too much, I would say that is enough to distrust it until proven otherwise.

My current hypothesis, which I could be wrong as I am still quite ignorant on this issue, is that your concentration of CO2 is simply determined by your flow rate/bubble count minus your off-gassing, regardless of any water chemistry. That is may be altered so slightly as to not matter by KH due to KH absorbing some of the .3% of CO2 converted to carbonic acid. That would need to reach an equilibrium and so perhaps a higher KH would cause a little more CO2 to be converted to acid vs free CO2 dissolved in the water.

Maybe a chemist is on board and can clarify this.

Last edited by ahem; Yesterday at 05:15 PM. Reason: added clarity
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post #8 of 9 (permalink) Old Yesterday, 06:40 PM
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Throw out the chart and you don't need a chemist.

To get an absolute measure of CO2 it takes expensive equipment. So we are left to our devices.

Which is basically controlling flow of CO2 to reach a relative pH drop. Absolute value really means little to us. And that goes for most everything in the hobby. It's relative values that count.

The trick is dialing in the right pH drop that brings out optimal plant healthy while not harming fish/fauna.

The "standard" folks talk about is a 1.0 pH drop. But optimal can be more or less depending on the tank.

A low light tank with slow growers needs less, a tank driven hard by light with lots of stems needs more.

In my tank (high light), my pH drop is 1.45. And you will find many other "high tech" tanks in the 1.2 to 1.5 range. The charts would tell you my CO2 concentration is 100+, and all the fauna would be dead, but in practice that is not the case.

So don't get too bogged down with the science. I've seen brilliant science minds that can't grow plants. Make choices based on what you see, not what you think or calculate should happen.
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post #9 of 9 (permalink) Old Yesterday, 06:48 PM Thread Starter
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I think you are correct. I am going to just focus on a stable bubble rate and the drop checker being green, adjusting if it becomes bluish or yellowish (trying not to err on that side). I think that's what the drop checker checks, green = -1pH I think.
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