Kh, Ph, & Co2 - The Planted Tank Forum
 
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post #1 of 7 (permalink) Old 01-18-2006, 04:59 PM Thread Starter
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Kh, Ph, & Co2

It seems to me that I read in a thread that chemically changing the PH of water could give false CO2 readings. Is this correct?

Out of the tap my PH is 6.6; out-gassed it is 7.2. KH out of the tap is 3.5. I had been adding baking soda to get the KH up to 4.5 to be safe, and pressurized CO2 injection took the tank down to a PH reading of 6.4. I am having some algae problems and it was suggested that I bump up the CO2. That creates a small problem in that my PH test kit goes from 6.4 to 6.0 and readings in that range are guessing at best. If I add some more baking soda I can take the KH up to 5.5 (also increasing the PH), which theoretically gives a higher CO2 ppm if I take the PH back down to 6.4 (a value I can read) through a higher bubble count.

Is this a good strategy or am I going in circles?
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post #2 of 7 (permalink) Old 01-19-2006, 10:25 AM
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Sounds like you're going about it right. There are certain products sold as 'buffers' which contain things that can skew the CO2 readings. But, the use of baking soda is certainly what I hear recommended continually as being a safe approach.
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post #3 of 7 (permalink) Old 01-19-2006, 05:37 PM
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Baking soda is safe because it changes the Kh of the water, which alters the pH proportionally.

Its when you start using phosphate buffers that alter pH but don't touch Kh, that the chart/relationship gets all outta whack.
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post #4 of 7 (permalink) Old 01-19-2006, 06:25 PM
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Why fix it if ain't broke? If your tap kh is 3.5, I wouldn't worry about raising it. You're fine. Get yourself another pH kit that will read in the range you need.

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post #5 of 7 (permalink) Old 01-19-2006, 06:55 PM
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Zelmo,

Most pH test kits have different colors that are 0.2 units apart. Maybe I'm partially color blind, but I find it difficult to tell two adjacent colors apart most of the time. That means that you really don't have any idea what the real pH is. You could be off by 0.2 or 0.4 units, even if your kit is perfectly calibrated, the test solutions are new, the comparison charts aren't faded, etc. I finally gave up and went with a pH meter. This might not be within your budget, but at the very least, get a better test kit.

Or, don't freak out too much about it. Turn the CO2 up gradually and watch the fish carefully. If they show signs of distress, back off a little on the bubble count. I think a KH of 3.5 should be fine if you don't ignore the tank for long periods of time. Once you get the parameters 'just right' it will still take a couple of weeks for new growth to overshadow what was formerly covered in algae.
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post #6 of 7 (permalink) Old 01-20-2006, 01:04 PM Thread Starter
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Quote:
Originally Posted by Bert H
Why fix it if ain't broke? If your tap kh is 3.5, I wouldn't worry about raising it. You're fine. Get yourself another pH kit that will read in the range you need.
I was worried that I was too close to the KH that would cause a PH crash when adding CO2. Can you tell me where that happens?
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post #7 of 7 (permalink) Old 01-20-2006, 01:25 PM
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The typical recommendation I see is you are safe so long as KH>=3. So, you should be good to go.
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