ph - kh - co2 tank water vs drop checker?

Davejt

Junior Poster
Dec 28, 2009
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Help me understand this...

Drop checker with 4dkh solution reads ph of 6.6 for 30ppm CO2

Tank tests at 4.5dkh with a PH of 6.0 (lower limit of my test). At 100+ppm CO2 but no fish are gasping this seems incorrect.

I will recheck tomorrow in case I contaminated a test.

I'd like the PH of my tank to be around 6.8 and up till now it has been. What am I missing here?

Additional info on request.

TIA
 

Davejt

Junior Poster
Dec 28, 2009
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Late/tired ... so should I up the KH in my tank to bring the PH up as the drop checker seems to indicate the correct CO2 levels?
 

Left C

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Sep 26, 2005
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A 4 dKH drop checker works because the only buffer in it is carbonate based. Aquarium water can't be used. The pH/KH/CO2 relationship is based on the fact that only carbonate buffers are used that effect the pH and KH.

Aquariums not only have carbonate buffers; they have phosphates which are strong buffers, tannins plus other bases and acids. All of these control the pH and KH of the aquarium. That is why you are thinking that you have such a high CO2 level and you really don't.

In Chuck's CO2 chart/calculator article, he mentions this and explains it.

"The pH-KH-CO2 Relationship: pH, KH, and CO2 have a fixed relationship as long as carbonate is the only buffer present (no phosphate buffers like pH-UP and- DOWN, Discus Buffer, etc). There are some parts of the country that have high levels of phosphates in their water supply. For those cases, determining CO2 levels will be difficult, as the phosphate will throw off the pH-KH-CO2 relationship, which means the CO2 charts and calculator below won't work. Note that the commercially available CO2 test kits will also be invalidated by the phosphates.

NOTE: This calculator (and the chart based on this formula) will only work if your water is carbonate buffered. If your water contains high levels of phosphates, it will alter your water properties, and invalidate these CO2 calculations."