# Ph profile, co2 and 0kh



## plantnoobdude (10 Aug 2022)

Hi, in my high tech 60p I maintain soft water with 0kh and also inject co2.

Though 1.0ph drop is generally regarded as optimal and safe for live stock, does this still apply for tanks with no kh? I recently checked my tank pH and degas pH. 

24h Degas sample is 6.65-6.7
Tank values stay from 
5.2-5.3 throughout the day so far.
This means I have a ph drop of 1.4!!! 

I was wondering, since I don’t have any buffer, is it possible I have less co2 in terms of ppm in the water, compared to say a tank with a kh of 4 with the same pH drop?

While my stock seem completely happy, I am a bit on edge. 

Thanks!


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## Zeus. (10 Aug 2022)

I use to run my 500l tank with a 1.4pH drop. But I wouldn't recommended it as there is a fine line between fish fine and opps
Lets assume the pH meters reading is correct, what colour is your DC m8 - mine was nearly clear at times


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## plantnoobdude (10 Aug 2022)

Zeus. said:


> I use to run my 500l tank with a 1.4pH drop. But I wouldn't recommended it as there is a fine line between fish fine and opps
> Lets assume the pH meters reading is correct, what colour is your DC m8 - mine was nearly clear at times


What was the kh of the tank? My fish seem quite at home and readily accept food and engage in regular behaviours through out the photoperiod.




Here is the dc colour, not yellow. 7 hours into photo period.


Zeus. said:


> Lets assume the pH meters reading is correct,


it is a Hanna ph meter. That has been calibrated and stays within 0.03-0.04 pH of calibration solutions.


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## plantnoobdude (10 Aug 2022)

Here is the general layout of the tank, co2 is provided via an inline diffuser.


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## Zeus. (10 Aug 2022)

I would trust the DC colour change as that is independent of kH and pH reading issues.
I think there also may be issues getting a correct pH reading when the kH is zero. I am not a chemist, but water can be tricky at times.


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## John q (10 Aug 2022)

plantnoobdude said:


> Though 1.0ph drop is generally regarded as optimal and safe for live stock, does this still apply for tanks with no kh?





Zeus. said:


> Lets assume the pH meters reading is correct, what colour is your DC m8 - mine was nearly clear at times


I reckon with low kh the ph drop is irrelevant, although I agree for safety reasons its always wise to go with a 1 point drop.
My degassed water sits at 7.3, tank water never goes above 7. When the lights come on Co2 drops ph to 6.1, so a 1.2 drop. DC goes from @plantnoobdude  green before Co2 comes on to a lime green/yellow at 6.1.

I'm paranoid about the dc colour and fish health, however I had a fault with solenoid timer a few weeks ago and Co2 stayed on all night, ph dropped to 5.8 and dc went yellow/clear, fish were completely fine, that's a 1.5 ph drop from degassed, assuming ph meter is correct.

What do we learn from this...  low kh water behavior is difficult to predict.


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## John q (10 Aug 2022)

I should add in a similar low kh tank PH hit 6.1 and this colour dc and I nearly killed all the fish..


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## jaypeecee (10 Aug 2022)

plantnoobdude said:


> Tank values stay from
> 5.2-5.3 throughout the day so far.



Hi @plantnoobdude 

Your pH is very low. Nitrifying bacteria may struggle at such a low pH. I can't determine what livestock you have in the tank. Please clarify. I have a couple of other observations...

Although your DC is green, it's a rather odd green. And the entrance to your DC seems to be partially blocked by 'froth', particles and bubbles. This may be affecting the DC colour.

Finally, your filter inlet seems to be partially blocked by plant fragments. This may be interfering with water flow throughout the tank.

Oh, one more question - what is your water GH?

That'll do for now.

JPC


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## plantnoobdude (10 Aug 2022)

jaypeecee said:


> Hi @plantnoobdude
> 
> Your pH is very low. Nitrifying bacteria may struggle at such a low pH. I can't determine what livestock you have in the tank. Please clarify. I have a couple of other observations...
> 
> ...


1. Indeed it is quite Low, though I doubt it is a problem. Stock is ember tetra and a honey gourami.

2. I have cleaned the dc and added a fresh batch of 4dkh water and bromothymol blue. Will report back in a few days.

3. I doubt it, the flow is very strong around the tank and no dead spots can be seen.

4. My gh is about 4 though I have had it as Low as 1.5 gh.


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## plantnoobdude (10 Aug 2022)

John q said:


> I should add in a similar low kh tank PH hit 6.1 and this colour dc and I nearly killed all the fish..
> 
> View attachment 192308


Not sure if comparable though, my pH is a almost a full point lower than yours.


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## plantnoobdude (11 Aug 2022)

Zeus. said:


> I would trust the DC colour change as that is independent of kH and pH reading issues.
> I think there also may be issues getting a correct pH reading when the kH is zero. I am not a chemist, but water can be tricky at times.


Ok, i will use the ph meter as a view of the co2 stability for now, and the drop checker to guesstimate the concentration.


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## Yugang (11 Aug 2022)

plantnoobdude said:


> Ok, i will use the ph meter as a view of the co2 stability for now, and the drop checker to guesstimate the concentration.


Using a pH probe at 0 kH  is not without risks  #2 as the logarithmic relationship between CO2 and pH is no longer valid (pH can go anywhere depending in water chemistry) as kH approaches 0. As said, best is using drop checker, as that is independant of tank water. Take any pH probe reading with a grain of salt. For just a check on stability seems that not much can go wrong.

Note: I am also at pH drops around 1.4, and livestock is perfectly fine with that. It does depend on the tank and especially O2.


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## _Maq_ (11 Aug 2022)

plantnoobdude said:


> Though 1.0ph drop is generally regarded as optimal and safe for live stock, does this still apply for tanks with no kh?


You need much more CO2 to reach 1 degree pH drop in water with higher alkalinity. And vice versa - just a few of CO2 injected is enough for the same pH drop in water with very low alkalinity. Therefore, 1.0 pH drop is perfectly safe in your tank. You're not overdosing CO2, rather the contrary.
(In fact, 0.0000 dKH is impossible as long as there's any CO2 present because it will always partly turn into bicarbonate as long as pH value is above 4.5.)


jaypeecee said:


> Your pH is very low. Nitrifying bacteria may struggle at such a low pH.


Yes, but it hardly matters because ammonia (NH3) will almost 100 per cent react with water creating harmless ammonium (NH4+). Still, my experience suggests nitrifying microbes function very well at pH above 6.0, and with diminished effectiveness even at pH around 5.5.


Yugang said:


> Using a pH probe at 0 kH is not without risks


Wrong. It does not depend on (bi)carbonate content but electric conductivity of water, in other words on general mineralization. Reading of pH gets difficult at conductivity lower than approx. 30 µS/cm. Your tank water is obviously far above this limit.

I've got quite rich experience with sparingly mineralized water. There are processes in any tank which may push pH both up and down. Sometimes I get quite perplexed about them.
Two situations seem to be particularly dangerous/unpredictable: young (non-matured) tanks and soil substrates.
I find this situation rather tricky. In my country, those who inject CO2 usually control dosing by pH (pH is the constant). But in this community, it is often stressed that amount of CO2 should be held constant. I don't know how your CO2 system works, and I've got no experience with CO2 injection, anyway.


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## PARAGUAY (11 Aug 2022)

Take a look at Green Aqua YT  were Tommy discusses testing. A lot of information there .


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## plantnoobdude (11 Aug 2022)

Yugang said:


> Using a pH probe at 0 kH is not without risks #2


I’m going to be honest, that is a very old post ( almost 20 years now) and I doubt Barr still agrees with what he posted now. 
Especially this part.
“_The system will crash, it might not have done this yet for you, but you'll get burned at some point. And have dead fish.”_


Yugang said:


> Take any pH probe reading with a grain of salt. For just a check on stability seems that not much can go wrong.


alright, I do calibrate it and it does to produce fairly accurate results though.


Yugang said:


> Note: I am also at pH drops around 1.4, and livestock is perfectly fine with that. It does depend on the tank and especially O2.


what is your kh? If you have a kh higher than mine and have the same pH drop I will be sure my fish are fine.


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## plantnoobdude (11 Aug 2022)

_Maq_ said:


> 30 µS/cm. Your tank water is obviously far above this limit.


I’m guessing that would be tds of 15 if you use a tds-500 scale.


_Maq_ said:


> Yes, but it hardly matters because ammonia (NH3) will almost 100 per cent react with water creating harmless ammonium (NH4+). Still, my experience suggests nitrifying microbes function very well at pH above 6.0, and with diminished effectiveness even at pH around 5.5.


Yes I agree, I even fertilize with ammonium nitrate and fish see no issues, if the tank were to have say a pHof 8. I’m sure they’d be less than happy….



_Maq_ said:


> I don't know how your CO2 system works, an


To clarify for everyone in this thread.
The co2 system is as follows;
Co2 supermarket dual stage reg with solenoid
Camozzi needle valve
Bubble counter
Qanvee co2 inline diffuser
Co2 drop checker.


_Maq_ said:


> You need much more CO2 to reach 1 degree pH drop in water with higher alkalinity. And vice versa - just a few of CO2 injected is enough for the same pH drop in water with very low alkalinity. Therefore, 1.0 pH drop is perfectly safe in your tank. You're not overdosing CO2, rather the contrary.


This is comforting😊

I guess I just have the tendency to over think things…. Green drop checker, stable pH throughout photo period and happy fishies and I should stop fretting about minor details.


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## Yugang (11 Aug 2022)

plantnoobdude said:


> I’m going to be honest, that is a very old post ( almost 20 years now) and I doubt Barr still agrees with what he posted now


I am not a chemist, but I doubt if insights on water chemistry, buffers and pH change a lot over time. It is true, we don't see many reporting on pH crashes in reality, but my guess is that some caution (that's why I said depends on tank chemistry) at 0 kH is appropriate. As I am around KH 2 (depending on season and sourcing of water) I am no where near the point where the discussion would be relevant for my tank


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## Yugang (11 Aug 2022)

_Maq_ said:


> You need much more CO2 to reach 1 degree pH drop in water with higher alkalinity.


This confuses me @_Maq_ . The famous KH/pH table is a log table, where every 0.1 pH corresponds roughly to a 25% change in CO2 ppm. 1 pH drop is roughly a 10 times increase of the CO2 ppm, irrespective of the KH. So if we assume that we start at about 3 ppm CO2 outgassed, a 1 pH drop will bring us to 30 ppm at any KH.

Where do I miss your point?


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## _Maq_ (11 Aug 2022)

Yugang said:


> Where do I miss your point?


If you inject CO2, a part of it will always react to produce HCO3-. The more so if your pH is higher. --> With higher alkalinity, it's more difficult to push pH down, be it CO2 or any other source of acidity (strong acid). Therefore, higher share will turn to HCO3-.
In ours, people tend to say that if alkalinity is higher than 5 °dKH, then injecting CO2 has hardly any effect. Obviously, this is not my personal experience.


Yugang said:


> if we assume that we start at about 3 ppm CO2 outgassed, a 1 pH drop will bring us to 30 ppm at any KH


Your assumption that 3 ppm is a fixed level is wrong. Yes, a 1 pH drop means _tenfold_ increase in CO2 concentration, but the starting concentration of CO2 depends on biotic factors and initial alkalinity.
An example: *a pH drop from 7 to 6* - if initial alkalinity is 4 °dKH, it means increase from 14 to 140 mg/L CO2. If initial alkalinity is 0.4 °dKH, it means increase from 1.4 to 14 mg/L CO2.


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## Yugang (11 Aug 2022)

_Maq_ said:


> An example: *a pH drop from 7 to 6* - if initial alkalinity is 4 °dKH, it means increase from 14 to 140 mg/L CO2. If initial alkalinity is 0.4 °dKH, it means increase from 1.4 to 14 mg/L CO2.


Fair, but perhaps we should keep the discussion practical.
When we mention pH drop in our hobby we usually mean with that '*pH drop as compared to outgassed aquarium water'*. And, without overcomplicating things, this is not really KH dependent and usually a good practical approach. We're not doing a PhD thesis here.



_Maq_ said:


> Wrong. It does not depend on (bi)carbonate content but electric conductivity of water, in other words on general mineralization. Reading of pH gets difficult at conductivity lower than approx. 30 µS/cm. Your tank water is obviously far above this limit.


If you would have quoted my full sentence, the argument would be entirely different. I am not referring to the workings of a pH probe. The point is that several credible refences  caution (I use a carefully chosen word here) about pH in water at 0 KH, and stress that the entire chemistry of the tank may lead to unexpected pH readings.


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## _Maq_ (11 Aug 2022)

Yugang said:


> We're not doing a PhD thesis here.


It's quite simple, though. One degree drop in pH means *tenfold *increase of CO2 content, but *you can't take for granted* that outgassed water contains 3 ppm CO2. It depends, among others, on alkalinity.


Yugang said:


> If you would have quoted my full sentence, the argument would be entirely different.


My apology, I've obviously misunderstood your point. Yes, I agree, a water with very low alkalinity is prone to unexpected changes in pH. Plenty of personal experience.


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## LMuhlen (11 Aug 2022)

_Maq_ said:


> It's quite simple, though. One degree drop in pH means *tenfold *increase of CO2 content, but *you can't take for granted* that outgassed water contains 3 ppm CO2. It depends, among others, on alkalinity.


The general idea of measuring degassed water pH is that we assume that the CO2 concentration is in equilibrium with the atmospheric CO2 concentration, and that doesn't depend on what is in your water. There is some discussion regarding this notion that this equilibrium is close to 3ppm, but that usually is because the atmospheric concentration could change in different indoor environments.

Now from what I understand of this discussion, the core of the discrepancy is the same that popped in my CO2 topic, if the DIC equilibrium applies only to one tiny fraction of dissolved carbon, the part that forms carbonic acid, or if it applies to the entire carbon population, making it so that high pH water in equilibrium has no CO2 and only carbonates and bicarbonates.


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## GreggZ (11 Aug 2022)

plantnoobdude said:


> While my stock seem completely happy, I am a bit on edge.


IMO no need to be on edge. Myself and many others have been running pure RO very little to no dKH tanks for years. There are loads of myths out there that still get repeated over an over. Most times it's from people who have never run a low dKH planted tank.

My fully degassed pH is 6.25 and I drop pH via CO2 injection to 4.85 on a daily basis. The readings are always very stable. My Rainbowfish show no signs of distress. However if I drop any much further they do begin to get lethargic.

That is the same at any dKH.  There is always a limit where fish will show distress. It varies a bit from tank to tank but I believe that has more to do with oxygen levels. CO2 and O2 are not mutually exclusive. The higher the O2 concentration the higher the CO2 concentration can be without affecting livestock.

The people I know who run tanks in this fashion do not experience a pH crashes. They have no problem with nitrifying bacteria dying. Fish do not die from osmotic shock.

And it's not to say that pH crashes are not real. They are almost always tied to very poor maintenance. A well run clean tank at zero dKH has nothing to worry about.

Here's an article from the 2 hr Aquarist site regarding low pH tanks. My tank is pictured from several years back when I was running 1 dKH. Shortly after that article I stopped dosing any carbonates at all and not a thing changed other than the degassed pH level.









						Is low pH in tanks due to aquasoils/softwater a concern?
					

Is low pH a concern in planted aquariums? Does the low pH caused by CO2 injection or aquasoil affect livestock or bacteria in the planted aquarium? Should I use buffers to raise the pH? This article examines why pH drops in aquasoil tanks and what should be done (or not done).




					www.2hraquarist.com


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## plantnoobdude (11 Aug 2022)

GreggZ said:


> IMO no need to be on edge. Myself and many others have been running pure RO very little to no dKH tanks for years. There are loads of myths out there that still get repeated over an over. Most times it's from people who have never run a low dKH tank planted tank.
> 
> My fully degassed pH is 6.25 and I drop pH via CO2 injection to 4.85 on a daily basis. The readings are always very stable. My Rainbowfish show no signs of distress. However if I drop any much further they do begin to get lethargic.
> 
> ...


My concern was in relation to co2 causing live stock stress, and not with ph crashes. 
None the less, thank you for your advice.


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## Hanuman (11 Aug 2022)

plantnoobdude said:


> Hi, in my high tech 60p I maintain soft water with 0kh and also inject co2.
> 
> Though 1.0ph drop is generally regarded as optimal and safe for live stock, does this still apply for tanks with no kh? I recently checked my tank pH and degas pH.
> 
> ...


No problems on this side of the earth.

1.4 PH drop from 6.38 down to 5 or slightly lower. 0 dKH reading. Everyone happy. Never been on edge. Drop checker yellow like urine. This said, if I had to wack a lot of plants from the tank I would slightly reduce Co2 injection to be safe.

Take a beer and relax.


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## Yugang (11 Aug 2022)

dw1305 said:


> You don't need to raise the dKH.
> 
> There are a number of difficulties with pH measurement and interpretation, pH is both a ratio and a <"log10 scale">.
> 
> ...


This is from a usefull thread.

(As a non-chemist, and happily at KH 2) I take it that in realistic situations in a tank (KH probably not exactly zero, substrate and all the other stuff), with fish as the ultimate judge, we should't worry  about CO2 injection at KH close to 0 as long we maintain the tank well. And of course all the usual precaution associated with CO2 as in any other tank.

For laboratory situations, less practical relevant, we have a case that pH does strange things at kH 0, and our usual logic using pH as a proxy for CO2 may not apply ('applies even less').


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## Hanuman (12 Aug 2022)

Yes, when people using RO say 0 dKH they mean they don’t add any carbonates either intentionaly through chemicals or through the addition of heavily loaded carbonate stones like seryu stones. Now this usually doesn’t mean there isn’t any carbonate in the water. The soil will actually contain some carbonates. If the tank had actually 0dKH I think you would not be able to see any snails and your PH would actually swing, but most dKH test are not sensitive enough to detect lower amounts of carbonates hence why we say 0dKH.


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## plantnoobdude (18 Aug 2022)

_*“Note 2: The pH-KH relationship is not linear. If your tank water measures 0-1dkH, you need to target around a 1.5pH relative drop to be within reach of the 25ppm CO2 target.”*_

From Dennis. This little tidbit of information is quite interesting, since it links my 1.4 pH drop to my green drop checker. Quite interesting, I wonder how he arrived at this conclusion. 









						Injecting Enough?
					

Above: Consistent and sufficient CO2 levels are central to vibrant, enduring Dutch-style 2Hr Tanks.If you have invested in CO2 injection, a good question is: am I injecting enough? If your drop checker is always green or if the bubble count is above, say 3bps, does it mean you have enough...




					www.2hraquarist.com
				



The article for anyone interested


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## Freshflora (18 Aug 2022)

plantnoobdude said:


> _*“Note 2: The pH-KH relationship is not linear. If your tank water measures 0-1dkH, you need to target around a 1.5pH relative drop to be within reach of the 25ppm CO2 target.”*_
> 
> From Dennis. This little tidbit of information is quite interesting, since it links my 1.4 pH drop to my green drop checker. Quite interesting, I wonder how he arrived at this conclusion.
> 
> ...


You can email the 2hr aquarist team or fb message him.  I’ve done both multiple times and he’s always responded.


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## Yugang (18 Aug 2022)

plantnoobdude said:


> pH-KH relationship is not linear.


The formula that I use is CO2 = 12.839 * dKH * 10^(6.37 - pH). Indeed the relationship pH to CO2/dKH is logarithmic, not linear.  It is important to understand that also outgassed pH is not a constant, but dependant on KH and ambient air CO2 concentration.
Most importantly, *a given pH drop (from degassed) will indicate an X% CO2 increase, irrespective of KH. (*pH drops 1.0, means 10 fold CO2 compared to starting CO2, from the above formula). This is also true for KH below 1, from all references that I've seen.



plantnoobdude said:


> if your tank water measures 0-1dkH, you need to target around a 1.5pH relative drop to be within reach of the 25ppm CO2 target.


This is really the first time that I read this, good catch @plantnoobdude , this is most likely not correct. Indeed good to reach out and ask for clarification.

Many hobbyists are around 1.4 drop ( I do 1.5 without any problem), and if fish are happy there is not much reason to worry. Fish could also be unhappy at a pH 1.0 drop, so what's in a number ... *Let the fish talk (and check drop checker as well)*  is what I learned

And for the plants .... *CO2 stability is what counts*, more than the absolute value. Unless we flood the tank with light, there is no confirmed benefit pushing the CO2 too high. This is important to remember, before we go into the rabbit hole of measurements and formulas.


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## Hufsa (18 Aug 2022)

Im not good enough with this kind of stuff to participate in the discussion, but I stumbled across something the other day that I thought might be relevant (be sure to click the link in that post to go to the thread)


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## _Maq_ (18 Aug 2022)

Yugang said:


> if we assume that we start at about 3 ppm CO2 outgassed, a 1 pH drop will bring us to 30 ppm at any KH.
> 
> Where do I miss your point?


@Hufsa has found this beautiful description: Cant get my head wrapped around the science


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## Yugang (18 Aug 2022)

Yugang said:


> the rabbit hole of measurements and formulas


See attached.

The formula I quoted above is what I found a couple of years ago, but when I search again not sure if there is consensus on that one. I used it for a model to estimate CO2 in my tank, and it was at the time good enough for what I wanted to do. The point is that absolute CO2 value matter less than stability, so why would we want to know precisely?

Simplicity first. Keep fish safe and CO2 stable


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## JoshP12 (18 Aug 2022)

Fish happy. Plants happy. This means bacteria happy. Water crystal clear? No need to worry. .

I am amidst rescaping my tank and debating removing all testing including DC and running co2 just by the system.


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## Yugang (19 Aug 2022)

_Maq_ said:


> @Hufsa has found this beautiful description: Cant get my head wrapped around the science


That post just plots out the well know logarithmic relationship. Basically what the CO2/pH table does, but then in a graph. 



Yugang said:


> We're not doing a PhD thesis here.


My view is that the topic is made unnecessarily complicated. Let me explain.

pH drop is a proxy for CO2. This means that a pH curve can be used to estimate what CO2 is doing, an indication of the absolute value and most importantly if it is stable. It is not a guarantee that the measurement is super accurate, indeed other chemicals can cause some perturbations and it is always possible to find situations where one should be cautious with conclusions.

Is it a problem using a proxy? Well, infact every measurement in science or technology is a proxy. There is no direct way to tell how tall you are, how old are fossils, or how far Alpha Centauri is from us. Basically these are all estimations based on observations, with many things that could potentially go wrong.

So back to the CO2/pH table. Yes it is a proxy, and chemists are happy to point out that things can go wrong and the measurement is not super accurate. The point is that the alternatives are worse. A professional CO2 probe is too expensive, so it is not of any practical value for most of us. And have you ever tried to accurately read out a drop checker to estimate what the weighted average of pH was over the last 3 hours?

So now back to basics. The CO2/pH table is the best proxy we have for practical purposes, and the general advice is to target 1.0 pH drop. It is not relevant for our practical aims that our proxy may be a bit off, as we don't care too much about absolute numbers but we care a lot about stability over time.

Most likely a (large) majority of high tech tank keepers are not using pH probes, and have therefore no idea how much instability they have in their tank. They tweak ferts and light, but the CO2 stability is the elephant in the room. The theoretical debate about the pH/CO2 relationship has much less practical value than a good focus on that big elephant in the room. 

In fact the theoretical discussion takes our eyes off the ball and confuses. Until we have an affordable and better alternative to pH probes and drop checkers.

(nothing personal @_Maq_ , I know you like a good debate )


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## PARAGUAY (19 Aug 2022)

JoshP12 said:


> Fish happy. Plants happy. This means bacteria happy. Water crystal clear? No need to worry. .
> 
> I am amidst rescaping my tank and debating removing all testing including DC and running co2 just by the system.


Why no drop checker Josh?


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## JoshP12 (19 Aug 2022)

PARAGUAY said:


> Why no drop checker Josh?


Always found it makes me second guess. Also found it distracts from the tank. 

Ex:
Fish looks lethargic but it’s green — no brainer turn it down … but it’s green so it’s fine? 

Ex:
Everything looks great! But it’s blue - gotta turn up co2. 

Caught myself down these rabbit holes too many times - including pH. 

Who knows I might re introduce it. 

But before I went hands off with my tank I found it to be much simpler: turn co2 on and look at tank, turn it up, look at tank. Fish unhappy, turn it down. 

Co2 with lights or 1 hour before lights. Lights just on or 30 min ramp. up and down. 

Tank still looks bad? Turn down ferts (use some starting point) and/or increase water change/maintenance frequency until you find balance. Lol

Going to try it and that means no drop checker, probes etc. .


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## Yugang (20 Aug 2022)

PARAGUAY said:


> Why no drop checker Josh?


Josh has a nice tank, so he knows what he is doing. For most of us it reminds me of:


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## PARAGUAY (20 Aug 2022)

That's me  @Yugang  one thing couldn't do without drop checker on CO2 tank.


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## JoshP12 (20 Aug 2022)

Yugang said:


> Josh has a nice tank, so he knows what he is doing. For most of us it reminds me of:



Tormented by obsession lol.


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## MrClockOff (26 Aug 2022)

Yugang said:


> The formula that I use is CO2 = 12.839 * dKH * 10^(6.37 - pH). Indeed the relationship pH to CO2/dKH is logarithmic, not linear. It is important to understand that also outgassed pH is not a constant, but dependant on KH and ambient air CO2 concentration.
> Most importantly, *a given pH drop (from degassed) will indicate an X% CO2 increase, irrespective of KH. (*pH drops 1.0, means 10 fold CO2 compared to starting CO2, from the above formula). This is also true for KH below 1, from all references that I've seen.


@Yugang  could you please elaborate with more details on the formula CO2 = 12.839 * dKH * 10^(6.37 - pH) please?
Please correct me if I'm wrong 

CO2 - achieved ppm concentration
*12.839 - what is this? Initial ppm of CO2 in the water*
dKH - carbonate hardness in degrees
*10^ what is this?*
6.37 - degassed water PH
pH - current PH


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## _Maq_ (26 Aug 2022)

@MrClockOff , leave the numbers as they are. 12.839 and 6.37 are constants.
(6.37 - pH) is an exponent - power.


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## MrClockOff (26 Aug 2022)

_Maq_ said:


> @MrClockOff , leave the numbers as they are. 12.839 and 6.37 are constants.
> (6.37 - pH) is an exponent - power.


Thanks @_Maq_ . Also found this https://fins.actwin.com/aquatic-plants/month.9707/msg00211.html


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## Yugang (26 Aug 2022)

MrClockOff said:


> @Yugang  could you please elaborate with more details on the formula CO2 = 12.839 * dKH * 10^(6.37 - pH) please?
> Please correct me if I'm wrong
> 
> CO2 - achieved ppm concentration
> ...


This formula  produces the same CO2 ppm data as the well known pH/KH table.

In my excel spreadsheet it reads =12.839*I$6*POWER(10,(6.37-F10)) , where cell I6 has the KH and cell F10 has the pH in it.

So indeed 10^ refers to a power of 10. The inverse is a logarithmic.
12.8 and 6.3 are constants, don't worry what exactly they mean other than that they are necessary for the formula. They may be a bit off, depending on the situation, which is not a major worry as we are more concerned with CO2 stability than with the exact value.

I used the formula, rather than pH/KH table, as I wanted to do some modelling calculations on pH profile. *For most, the pH/KH table is an easier tool*. I quoted the formula above because

it illustrates so clearly that a 1.0 pH drop always (irrespective of KH) represents a 10 fold increase of CO2 ppm. (This is a exponential/logarithmic relationship)
it illustrates how 10% KH change (at constant pH) represents a 10% CO2 ppm change. (This is a linear relationship)
Once we have this high level understanding what is actually in the pH/KH table, we don't need to reference it that often any more. And, relevant to this thread, the relationship between CO2 and pH does not fundamentally change when KH is low (although strange things happen when KH is exactly, mathematically 0).


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## MrClockOff (27 Aug 2022)

Awesome explanation! Thank you @Yugang


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## dw1305 (27 Aug 2022)

Hi all,


Yugang said:


> it illustrates so clearly that a 1.0 pH drop always (irrespective of KH) represents a 10 fold increase of CO2 ppm. (This is a exponential/logarithmic relationship)
> it illustrates how 10% KH change (at constant pH) represents a 10% CO2 ppm change. (This is a linear relationship)


We have an old thread somewhere. I'll see if I can find it. This one:

@Jose comments in this one <"Question about pressurised CO2 and water disturbance">.

cheers Darrel


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