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.

Does the kH drop significantly with CO2 injection? My understanding was
that the pH dropped with CO2 injection because the kH didn't. On the
other hand, CO2 makes carbonic acid which would drop your kH.

No, CO2 doesn't touch Kh it reduces Ph


I thought the NH4 to NH3 conversion was a direct function of only pH.
Under acidic forces, the kH will indirectly alter the NH3 to NH4 by
allowing the pH to drop (but I'm no chemist).

I'm no chemist either (!) I just know that at low levels of Kh you can run
into OTS if you arn't careful - its not likely to be an issue if the water
you use for changes has the same Kh as the water in the tank - its when the
new water is much harder that the problem emerges AIUI. Hence why "Old tank
syndrome" tends to happen to people who use tap water in their tanks & don't
change any for ages.

I.