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Re: CO2 vs. KH testing

Dave Berryman wrote:
> After water change
> KH              2
> PH              7
> 1 hour after adding 2 tspn baking soda
> KH              4
> PH              7
> 2 hours after adding another 2 tspn baking soda
> KH              6
> PH              8
> Notice the rise in PH?  Lights had been off for an hour by then so can that
> account for the rise in PH?


> So I have now proved to myself that the KH is in no way directly affected by
> the swing in PH when the CO2 is altered and that the PH is in no way
> directly affected by the swing in KH when the KH is altered.


I'm glad you convinced yourself that KH isn't effected by pH, but I
don't follow your conclusion that pH isn't effect by KH.

Using the KH-pH-CO2 chart, you had about 6 mg/l in your tank after the
water change.  You increased KH from 2 to 4 and the pH stayed at 7.  The
CO2 level increased from 6 mg/l to about 12 mg/l.  The CO2 level may
have gone up just because it was recovering from the water change.  Then
you raised the pH from 4 to 6 and the pH increased from 7 to 8 (that's a
large change); the CO2 for some reason dropped to less than 5.  I don't
have a clue why your CO2 level would drop so much.

If you kept the CO2 level constant at 12 mg/l, then increasing the KH
from 4 degrees to 6 degrees should have increased the pH from 7 to about
7.15.  An increase, but not a big one.

> Which brings
> me to two more questions...how high is to high for the KH?  And should I
> bring the GH up also?

I don't know of any practical upper limit for KH.  My tap water is about
7 and other people have more.  The lower limit is more important.  You
probably want to have 3 or 4 degrees to avoid problems.  In many natural
waters the alkalinity (KH) and hardness (GH) are about the same.  It
isn't necessary for you to keep them equal, but for the plants' sake you
probably should try for at least 3-4 degrees GH.

Roger Miller