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CO2 levels and KH

on 01:48 PM 9/28/99 , Aquatic Plants Digest wrote:

 >From: "wayne jones" <waj at mnsi_net>

 >I thought it was settled that kH has
 >nothing to do with pH variation due to
 >CO2 injection. I take this to mean that
 >if an airstone drives off most of the
 >CO2 and the pH does not move very much
 >then you did not have much CO2 in the
 >first place. I am not saying your
 >observations are incorrect just that
 >they appear to contradict my
 >understanding of the theory.

I'm still confused about this. See for instance this article at the Krib:


Here's an excerpt:

 >The higher the amount of carbonates/bicarbonates in the aquarium, the 
more CO2 is needed
 > to maintain a specific pH.

I know this is talking about a specific pH rather than a specific change in 
pH, but it still doesn't jive with what I've been reading lately on the 
APD, or for that matter with my real-world experience.

Here's a question: Does it require more CO2 to change the pH, say, from 8.0 
to 7.0, than it does from 7.0 to 6.0? Would that explain my pH's seeming 
reluctance to change at pH 7.8 and 7 dKH?

The other question is, where is my CO2 going? I have one bubble per second 
coming through an Eheim diffuser, which in turn is right below my Fluval 
intake. As far as I can see *every* bubble is being dissolved. This should 
really be more than enough CO2 for a 29 gal tank, right?

But my pH only changes from approximately 7.8 to 7.6. By the chart, that 
gives me between 4-6 ppm CO2. Shouldn't I have more than that? There is 
little to no surface agitation.

Should I crank up the CO2 and try for a significant pH change? Or does an 
alkalinity of 7-8 dKH really make it hard to change the pH with CO2? From 
the chart, it seems like bringing the pH down to 7.0 with CO2 would give me 
a nice 20 ppm, but I'm curious just how much CO2 that would take.

According to the chart, at KH 0.5, adding 15 ppm of CO2 would bring the pH 
from 8.0 down to 6.0. At a KH of 8.0, the same 15 ppm would take the pH 
from 8.0 down to about 7.1. Is this an inappropriate use of the chart? It 
seems to me that this says that CO2 changes the pH less at a higher KH...

michael moncur   mgm at starlingtech_com   http://www.starlingtech.com/
"My one regret in life is that I am not someone else."    -- Woody Allen