"Kris" wrote in message
news:bebzc.18815$lN.13766@edtnps84...
The two are linked. At a known level of CO2 (i.e. what you get from the
Atmosphere) as KH falls so Ph will fall with it. In other words if you
want
to bring down the Ph to 7 then you need to reduce the Kh which you can
do
with Peat, or bogwood etc.

However, two issues...

a) As Kh approaches zero so Ph becomes unstable and can crash (to 5 &
below)
with disasterous results.

I do and don't agree with the above statements.

IIRC
a high KH (70 ppm) will raise pH, but KH, within normal ranges, acts as
a stabilizer (buffer) for pH, the lower the KH the more drastic the pH
swings maybe. An average KH 30-70 ppm will keep large (rapid) pH swings
from happening due to outside factors. A KH of lower than average can
allow pH swings caused by something as simple as the aquariums lights
on/off cycle.

You are right in that the relationship between Kh & Ph is not linear
(there's a log in the formula somewhere!) - I've got a spreadsheet that
works it out somewhere. Kh has to get pretty close to zero before Ph starts
to really crash but as it gets closer the decline in Ph definitly
accelerates. The OP was talking about wanting a Ph of around 7 - according
to what I have at atmosphesric levels of CO2 (3ppm roughly) that would
require a Kh of about 1dKh (15-17ppm) which is still stable & maintanable
but doesn't leave much room for error - its getting pretty close to zero
IMO.

I.