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Re: [APD] Aquatic-Plants Digest, Vol 29, Issue 57

> Date: Tue, 31 Jan 2006 10:54:31 -0800
> From: Vaughn Hopkins <hoppycalif at yahoo_com>
> Subject: [APD] CO2 Measurement Method
> To: Aquatic-Plants at actwin_com
> I have a cork wall in my 29 gallon tank.  I also have been
> measuring 
> 150 - 250 ppm CO2 using PH/KH table.  Being somewhat dim
> witted it has 
> taken far too long for me to connect those two facts. 
> Apparently cork 
> leaches tannins into the water which drops the PH a bit,
> making the 
> PH/KH/CO2 table inaccurate.  Today I checked a water sample
> taken from 
> the tank last night and allowed to sit over night vs. one
> taken this 
> morning.  Last nights sample had KH of 9 degrees, PH of 7.0 -
> 27 ppm 
> CO2.  Todays fresh sample had KH of 9 degrees and PH of 6.2 -
> 170 ppm 
> CO2.    I think this strongly indicates that the cork is
> fouling up the 
> CO2 measurement.   But, the .8 drop in PH should mean my
> actual CO2 is 
> about 25 - 30 ppm, which the fish and plant reactions say it
> should be. 
>   Any comments?

Several useful tips: 

1.You can estimate based on the relative pH.
Say the pH of the water that sat out for 24hrs has a pH of 6.8.
Add enough CO2 to drive the pH to 5.8-5.8 or so.

2. Test the tap water's KH. Assume the tank's KH is the same and
do a series of large water changes. Add enough CO2 to drop the
pH to the desired level. If the pH drops and the rate of CO2 is
steady(say the bubble rate), then do not worry about the drop,
the rate of CO2 dosing is still the same as is the plant uptake
of CO2.

So you can use non tannins water to dial in the CO2, then allow
the tannins back in or soften the water to a KH of zero ppm etc,
and still provide CO2 to the plants effectively.

Most test kits measure total alkalinity, not bicarbonate
alkalinity. Electronic pH meters often have skewed measurements
due to stray voltage, turn things off like lights etc before
taking a reading.

Tom Barr



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