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Re: How low can I keep my KH?

> Date: Sun, 20 May 2001 05:08:00 -0400
> From: Jared Weinberger <jared at brainyday_com>
> Subject: How low can I keep my KH?
> I know some, like Paul Sears, have said that one's KH can actually be kept
> lower than the 3-4 generally advised without risk. I would like to know
> just how low I can keep our KH given our setup:

	I've got a bit behind following the digest (and it will probably get
worse).  The point I would like to make is that if the only acid going
in is CO2, then KH 1 is just fine.  KH 0.1 will give a still lower pH (by 1)
and may also be fine.  The snag arises if a strong acid gets added (like
nitric, from ammonia oxidation).  The higher KH numbers give you a margin
of safety, because strong acids reduce the KH.  KH 0.1 may give an
acceptable pH, but the amount of strong acid required to reduce it to
KH 0.00001 is very small, and accidents can happen.  If you are using
low KH, keep an eye on it.

> - -- Tap KH is 24 ppm (1.3 dH). I've been using sodium bicarb. to keep this
> at about 30-40 ppm. pH has been about 6.9. Is there really a risk of a
> drastic pH drop with a 20% daily water change?

	No.  If you reduce the KH by a factor of 1.5, the pH will drop
by about 0.18.  You have just given away about 30% of your margin for
addition of strong acids.

 It would like to keep the pH
> and CO2 low with this naturally low KH. Given daily water changes with tap 
> with this KH, how low can I safely keep my KH? Is 1.3 too low? With 
> supplementary CO2, does alkalinity (carbonate) serve any purpose other than 
> pH stability? 

If you keep doing 20% daily water changes, I would think that KH 1.3 
would be quite adequate, since you are protecting yourself from major 
KH reductions by adding more each day.  Keep measuring what you have 
in the tank.  

You must have _some_ buffer system in the water, and 
HCO3-/CO2 is the obvious one to use.  

Paul Sears        Ottawa, Canada