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I've been running a 29 gallon tank with 2 55watt compacts and a carbo plus co2 system for over a year with mixed results. I know that the Carbo Plus is probably part of the problem and I'm getting a new High Pressure co2 system soon. While my tank has looked beautiful at times I have never had the tall fast growth that this amount of light should produce. I use an Aquarium Pharm tap water filter for water changes and add Kent r/o right and Kent Botanic KH+, but my KH always measures under 2˚.
I have long suspected that the co2 is not being absorbed into the water because of a lack of a KH buffer. Does anyone have any suggestions for a good stable additive to get the KH up to 4˚ without sending the pH through the roof? Are the KH and pH linked? Will the raised pH drop with co2 addition? It doesn't seem to with the Carbo Plus.
I'm looking forward to your comments. Thanks.
 

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IIRC, the carbo plus actually uses the carbonates in the KH of the water. This is likely why you see a reduction in the KH of your tank.

Baking soda has long been used to add KH to the water. It will have an effect on the pH by bringing it up a little.
 

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Hi Dave,

Thanks for supporting APC!

The relationship between pH, KH and CO2 is quite complex. I'll try and summarize it here using a quote from someone on the APD quite a long time ago.

Changing pH necessarily involves changing KH, CO2 concentration or both. The 3 parameters are related via the Henderson-Hasselbalch equation:

pH = pKa + log([HCO3-]/[H2CO3])

where pKa is a constant (6.35 in FW), [HCO3-] is bicarbonate ion concentration (which is essentially equal to KH in FW), and [H2CO3] is effectively the dissolved CO2 concentration. This equation holds irrespective of whether other buffers are present or not.

Some methods of changing pH directly target KH (e.g. adding baking soda) or CO2 (injecting). Other methods also change KH (and CO2 in the short term) but do so slightly less directly and hence wouldn't necessarily be thought of in terms of manipulating these variables. For example, adding NaOH to increase pH wouldn't normally be described as increasing KH or reducing dissolved CO2, but that is nonetheless what happens. The removal of H+ ions by combination with OH- ions causes more H2CO3 to ionise, reducing dissolved CO2 (temporarily) and increasing HCO3-. This increases the numerator and reduces the denominator of the log term in eq.1 and allows the equation to balance at a higher pH. The reduction in CO2 (and most of the increase in pH) is temporary because the long-term equilibrium level of CO2 in an aquarium is mainly determined by physical and biological processes (exchange with the atmosphere, respiration and photosynthesis), and the end result when all the dust has settled is a (very) small increase in both pH and KH.

So it is true that changing pH is synonymous with changing KH (or CO2 concentration), but perhaps sometimes a bit misleading to put it that way, especially if another buffer is introduced.


Additionally, it may be helpful to use the pH/CO2/KH table as your reference as you try to target a specific CO2 concentration. I've attached it below.

How are you reconstituting the purified water to add back KH?
 
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