Re: pH, KH and CO2

> From: stevensj at calshp_cals.wisc.edu
> There's a water FAQ that states that for a planted tank pH 6.9, KH 4 and
> CO2 15 ppm is just about ideal.  I'm wondering if others aim for this like
> I've been trying to do.  

Yep, this is what we shoot for. 

> I think I have a problem with either my pH or KH
> test kit though because at KH 4 (tetra test kit), CO2 15 ppm (La Motte)
> my pH is reading 6.4 (Red Sea fish pHarm).  If I increase CO2 to 18 or
> higher my pH drops below 6, even with extra teaspoons of baking soda.
> This is in a 25 gal tank.

First, be aware of possible error ranges when you make your
measurements.  If you use "5 ml" of water for the tetra KH test, 1
drop is 1 dKH.  Your error may be +/- 1 dKH depending on how you
interpret the color change.  I suggest using "10 ml" of water so that
1 drop is 0.5 dKH.

I'm not sure how accurate your pH test kit is, but an error of +/- 0.4
seems possible with some I have seen.  

The CO2 kit is good for +/- 2 ppm, IMHO. 

So it's not difficult to get readings that seem wrong.

Also, be aware that the tetra KH kit is really measuring alkalinity
and not just "carbonate hardness".  If you have other buffers in the
water, like the commercial pH-UP and -DOWN, they will add phosphate
buffers will will add to the alkalinity and cause the "KH" kit to read
too high.  If most of the alkalinity is due to carbonates (like from
baking soda), then the Tetra kit is just fine.

> Anyone recomend any better test kits?

I would recommend the Lamotte "narrow range" pH test kit (6.4 to 7.5).
The narrow range kit is harder to find (most places stock the wide
range kit).  The narrow range kit lets you get to 0.1 pH units.  The
other kits you have are just fine.