"NetMax" wrote in message .. .
[snip]
AFAIK, caustic soda has no effect on gH or kH, only pH, and it's not
particularly stable.
[snip]

Caustic soda is NaOH. While it doesn't directly affect kH, it will
indirectly raise it.

Once the water is out of the tank, the concentration of CO2 will come
to some equilibrium. If your pH is high due to NaOH addition, some of
the CO2 the water absorbs will be converted to bicarbonate. The water
will keep absorbing CO2 until the kH is at the equilibrium level for
the pH and CO2 concentration. During this process the pH may come down
a little.

In other words, at a given CO2 concentration and pH level, there's
only one stable amount of bicarbonate. So, given a little time, your
water will get there.

I've just started to add a bag of crushed coral to a filter compartment
to see if I can boost the kH slightly while keeping the pH in the low 7s/
high 6s. It's an experiment to see if I can achieve some greater
stability through non-chemical means. My test application is a high
fish-load, 1/3 planted, non-CO2 tank running 50% w/c per week (almost
continuous w/c system). If anyone has any other ideas on how to
counteract caustic soda without stripping my last few remaining
carbonates, I'm all ears.

For the reasons I just explained, you can't. Adding CO2 is the only
possible mechanism to reduce pH without reducing kH. This is because
the pH is set by the ratio of CO2 to bicarbonate. So if you reduce pH
without adding CO2, you'll reduce kH. If you increase pH without
adding bicarbonates, you'll quickly get more bicarbonates anyway as
atmospheric CO2 gets converted.

- Jim