Lowering KH - The Planted Tank Forum
 
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post #1 of 7 (permalink) Old 11-22-2006, 04:51 PM Thread Starter
 
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Lowering KH

After wrestling with a new ph meter and now with a reactor...I finally got my ph to come down. However, I have had to run my CO2 (pressurized JBJ system with PGO reactor) at a fast rate. To much flow to really count any bubbles. At that rate I got my ph to drop from 7.6 to 6.7 ...but...when it got to 6.7..I seen a lot of stress on fish. I right away tested the KH to find that it had not lowered what so ever! According to the sticky and the chart provided...the 289ppm KH vs. 6.7ph was a dangerous combo for the fish...so I turned back the CO2. The ph went back up to 7.4.

At what point does the KH also drop???? Does CO2 also lower KH?? Any help would be appreciated
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post #2 of 7 (permalink) Old 11-22-2006, 05:12 PM
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No, without getting into the chemistry too much, CO2 does not lower KH. Because H2CO3 (carbonic acid), is an acid, it is assumed that it will lower the KH. This is not true as only the chemical species change which provide alkalinity.

H2CO3 --> H+ + HCO3-
HCO3- --> H+ + CO3--

The pH will change, but the KH remains the same. The net change is zero.

KH drops when a acid with a pKa (dissolution constant) greater than carbonic acid is added to the water from fish metabolites, bacteria, decay, etc. This lowers the KH as it exhausts the ability of the primary 'buffers', e.g. carbonate, bicarbonate to accept any more protons....(H+), called the carbonic acid equivalence point.


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post #3 of 7 (permalink) Old 11-22-2006, 06:39 PM Thread Starter
 
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So, I must keep my ph at a agreeable level with the KH..or I will endup with dead fish. But, by maintaining these levels...is the amount of CO2 going into the tank that beneficial to the plants?
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post #4 of 7 (permalink) Old 11-22-2006, 06:48 PM
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Quote:
is the amount of CO2 going into the tank that beneficial to the plants?
Yes....CO2, most recommend around 15 -30ppm. That is a safe range for fish, and a good level for plants to start with.

You can figure the amount of CO2 in your tank with this calculation:

CO2 = 3 * KH * (10^(7-pH))

Substitute your KH and Your pH.


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post #5 of 7 (permalink) Old 11-22-2006, 11:52 PM
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A low pH will not kill your fish. That would only happen if you dropped the pH down below 3 or 4, and you won't do that with CO2. If you add too much CO2 and don't reduce it pretty quickly that can kill the fish by suffocating them. I am using 40 ppm of CO2 now, with no fish problems at all, so at least that much is ok.

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post #6 of 7 (permalink) Old 12-23-2006, 06:10 PM
 
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Just to also add, obviously a sudden drop in pH will have a impact on your fish. So if you pH dropped quickly, then this may also explain why your fish were not happy.
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post #7 of 7 (permalink) Old 12-23-2006, 07:28 PM
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Agreed, You probably want to slow the rate of CO2 input so the pH shift is a bit more gradual. One way to do that is to put the CO2 on a timer. Have it come on a couple of hours before the lights do, and off around the time the lights go off, or even an hour before the lights go off. But at a slower bubble rate than before...

FWIW, there is a lot of fluctuation in a natural setting too, but with a slower shift, it is less stressful to the fish.





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