# Drop checker color



## 691175002 (Apr 28, 2009)

It depends on the dKH of your drop checker solution.

Essentially you use these two charts:


















Normally people use a 4dKH solution and aim for 20-30ppm of CO2 which means your color should be in the 6.8 to 6.6 range which is approximately lime green.

There is are a lot of steps to go from CO2 concentration to a color, and most of them are very approximate. The color is only useful for a very ballpark estimate.


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## clopez1 (Jun 30, 2013)

Thanks for the information. Just trying to be careful. Installed the DC yesterday and it turned pale yellow, would hate to go from too much to too little.

Bump:


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## klibs (May 1, 2014)

watch your fish / plants / algae. Many will say it is the best and only true way to test CO2 levels. By design / function drop checkers are certainly not an exact science. All they really do is detect changes in pH.


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## exv152 (Jun 8, 2009)

If you used a 4ºdKH solution, then green is good, yellow is too much, and blue is insufficient co2. Personally I aim for more than 20ppm, more like 30-40ppm, which is still safe.


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## mattinmd (Aug 16, 2014)

klibs said:


> watch your fish / plants / algae. Many will say it is the best and only true way to test CO2 levels. By design / function drop checkers are certainly not an exact science. All they really do is detect changes in pH.


If you use aquarium or tap water, I understand this is very true. 

If you use a a distilled water + carbonate only solution of known dKH (ie: 4dKH), then the measurement of CO2 should be accurate. Right? 

At that point there are no non-carbonate buffers present inside the drop checker, so the pH vs CO2 levels and should match up exactly with the charts.

Using tap or aquarium water in the drop checker is likely to introduce non-carbonate buffers, which throws things off quite a bit, particularly if there's phosphate buffers in the water.

Or am I missing something here that further reduces accuracy of drop checkers?


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## 691175002 (Apr 28, 2009)

mattinmd said:


> Or am I missing something here that further reduces accuracy of drop checkers?


You are pretty much right, but the small errors will start to add up.

If your 4dKH solution is out by 10% your measurement will already be off by a bit.

Judging pH by eye is very difficult, especially in the aquarium where the lighting and tannin content of the water throws things off. Differentiating 6.6 and 6.4 is pretty damn hard, even with a color chart. Doing it in the tank without a reference is nearly impossible.

Miss the dKH by just 0.1 and the pH by 0.2, and your calculated CO2 could be off by 20ppm or more.


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## krissyau (Jul 2, 2020)

*Help please!*

Hello,

I am confused how to know which is the correct way with drop checkers too.

I have a 130L (34.3 gal) freshwater community. Ph is 7.4. KH is 6. GH is 7. Ammonia and nitrites 0. Nitrates looks 10. Co2 running 1.5 - 2 bubbles per sec.

Firstly, I bought a brand of co2 checker here in Australia called dymax. Instructions say to put 5 drops of supplied solution and 1.5 ml aquarium water. This immediately went blue, and didn’t change colour at all in my tank the week it was in there

Then I cleaned it out and put 5 drops solution and 1.5ml of demineralised water we can buy here at our grocery stores. It immediately went yellow, and stayed yellow the 2 weeks I had it in my tank

I looked on the chart (as above) and it says blue 9. What does that mean exactly? 

What water am I supposed to put in the checker with the solution? On websites that sell this brand checker, they say to just fill half way with solution only.

Very confused as what to do...can someone please advise :crying:

View attachment 903741


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## Darkblade48 (Jan 4, 2008)

krissyau said:


> Hello,
> 
> I am confused how to know which is the correct way with drop checkers too.
> 
> ...


The solution that was supplied is likely bromothymol blue (an indicator solution).


It will change colours to indicate (roughly) the amount of CO2 that is in your aquarium.


However, in order to use a drop checker properly, you cannot use aquarium water nor distilled water. You must use a 4 dkH reference solution (where the only contributing carbonate hardness species are bicarbonate/carbonate anions). You need not worry too much about the chemistry behind it, but you will need to find a 4 dkH reference solution (or you can make one up yourself, if you have access to a scale that can measure to at least one decimal place).


Hope this helps


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## en7jos (Jun 7, 2020)

Darkblade48 said:


> However, in order to use a drop checker properly, you cannot use aquarium water nor distilled water. You must use a 4 dkH reference solution (where the only contributing carbonate hardness species are bicarbonate/carbonate anions). You need not worry too much about the chemistry behind it, but you will need to find a 4 dkH reference solution (or you can make one up yourself, if you have access to a scale that can measure to at least one decimal place).


Well I didn't know that! Is this 100% correct in all cases? I have the same/similar Dymax indicator solution mentioned above and the instructions say "Add 1.5ml of *aquarium water* into the indicator". The other solution I have (Ista) you don't dilute at all, just use as is from the bottle. But that never seemed to give a clear indication, so I tried the Dymax "dilute with tank water" solution which seems much better. My tank water is around 3-4 dKH though, so maybe that's why it works ok for me?

Just noticed that there are two colour reference charts on the side of the Dymax solution box, one for 2-7 dKH and one for 8-15 dKH. Is this how they get around the using tank water complication then?


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## Darkblade48 (Jan 4, 2008)

en7jos said:


> Well I didn't know that! Is this 100% correct in all cases? I have the same/similar Dymax indicator solution mentioned above and the instructions say "Add 1.5ml of *aquarium water* into the indicator".



Yes, the drop checker relies on the pH/kH/CO2 relationship (charts of which can be easily found online). However, what these charts fail to mention is that this relationship is only true if you are using a known dkH reference solution, of which bicarbonate/carbonate are the only contributing anionic species.




en7jos said:


> The other solution I have (Ista) you don't dilute at all, just use as is from the bottle. But that never seemed to give a clear indication, so I tried the Dymax "dilute with tank water" solution which seems much better. My tank water is around 3-4 dKH though, so maybe that's why it works ok for me?


The Ista solution you have may be a 4 dkH reference solution pre-mixed with bromothymol blue. However, this may not necessarily be the case, given that you are reporting problems with it.


If you are using aquarium water that is roughly 3-4 dkH, then the drop checker will work, but will not be as accurate as it could be (there are other variables inside aquarium water that can affect carbonate hardness, thus throwing off the readings of the drop checker).



en7jos said:


> Just noticed that there are two colour reference charts on the side of the Dymax solution box, one for 2-7 dKH and one for 8-15 dKH. Is this how they get around the using tank water complication then?


They may be trying to get around the aquarium water carbonate hardness with this scale (e.g. one for lower carbonate hardness, one for higher). However, as I mentioned, this suggests that they are asking you to use aquarium water, which will not necessarily provide an accurate result regardless (though it is convenient).


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## jellopuddinpop (Dec 12, 2016)

mattinmd said:


> am I missing something here that further reduces accuracy of drop checkers?


The biggest issue with using only a drop checker to check your CO2 content, is that there is a significant delay between the CO2 content of your water and the color displayed in the drop checker. It’s generally accepted that your drop checker will show you what the CO2 content was about an hour and a half ago. This is because the CO2 in the tank water needs to offgas into the small air pocket in the drop checker, then reabsorb into the drop checker solution. It's like time travel in a way. You can only see what your ppm was, but never what it is.


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## ahem (Dec 27, 2014)

I'm new to CO2 also. I can't tell you how much peace of mind this modest costing piece of equipment has brought, a $125 pH controller.

It's nice to see real-time where you are and to be able to precisely stay within your 1 pH value drop. It's even nicer to know it will automatically switch off the CO2 if it goes below your min value rather than titrating to the point where "your fish show distress". I target a .7 pH drop to stay on the safe side. Even at that level stuff is growing so fast I have a lot more trimming than I thought I would.

The drop checker appears to be a reasonably accurate measurement, I keep as a back up. It's two main problems are that it's slow, shows an hour or so behind supposedly, and you have to keep it maintained. Water slowly creeps in, fluid needs changing, and always worried of a contamination or shrimp crawling in. 

I've learned CO2 has a lot of variance and fuzzy information out there. Perfect example is the popular chart in #2 post on this thread appears incorrect. (came up in another thread). It shows a higher PPM as "too little" CO2 and low PPM amounts as "too much." There are different versions of this chart floating around that show the opposite. I like the Dymex simplified version!


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## en7jos (Jun 7, 2020)

Darkblade48 said:


> Yes, the drop checker relies on the pH/kH/CO2 relationship (charts of which can be easily found online). However, what these charts fail to mention is that this relationship is only true if you are using a known dkH reference solution, of which bicarbonate/carbonate are the only contributing anionic species.


Thanks @Darkblade48 for the explanation and info, much appreciated roud:

So next question is: How do I make a _"known dkH reference solution, of which bicarbonate/carbonate are the only contributing anionic species"_ ?

I have a pot of Seachem "Alkaline Buffer". If I remineralise RO water to 4 dKH using this, is the a good enough ref solution? Sorry, never did chemistry and the bit about bicarb/carbs and anionic species goes way over my head!

Thanks, James


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## Darkblade48 (Jan 4, 2008)

en7jos said:


> Thanks @*Darkblade48* for the explanation and info, much appreciated roud:
> 
> So next question is: How do I make a _"known dkH reference solution, of which bicarbonate/carbonate are the only contributing anionic species"_ ?
> 
> ...


 Instructions are here:


https://www.plantedtank.net/forums/26-planted-tank-faq/42429-kh-standard-how.html


Use the purest anhydrous sodium carbonate or sodium bicarbonate that you can find. Regular sodium bicarbonate (used for cooking) can work, but it might have other stuff in it.


Best practice is to use class A volumetric glassware with reagents of known purity (purchased through Sigma, VWR, etc). Of course, these aren't generally accessible by most people, so using the above instructions is fine for our purposes.


That being said, if you have access to the proper equipment, by all means, it is best to use it


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## en7jos (Jun 7, 2020)

Darkblade48 said:


> Instructions are here...


Thank you! Added to my to do list :grin2:


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