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Julius wrote:  "Someone mentioned that the CO2 dropped the PH to a level
that made their fish happy. This is not a safe practice. If you have a power
outage and your solenoid valve shuts off, the PH can rapidly jump putting
stress on the fish. It's a bad idea to regulate PH with CO2.  If you need to
lower your PH, use RO water and then use a buffer to maintain a certain

This statement worried me and I thought it might mislead folks who don't
understand the relationship among CO2, pH and KH.  CO2 does affect pH in all
cases in a completely understandable and predictable way.  CO2 in
conjunction with  KH does in fact "regulate" pH.  Full stop and in all
cases. Nothing could be worse that using RO water and setting your KH to an
arbitrarily low level (say below 2 degrees KH) and then ignoring the affect
additional CO2 will have on pH.  CO2 will lower pH in every case based on
the amount of carbonate hardness (KH) present in the water column.  If you
supplement CO2 as many of us do, there is no getting around the fact that if
the CO2 shuts off, the pH will rise.  How much is a function of KH and the
rate at which the tank system reaches equilibrium with the air (2 - 3 mg/l)
when the CO2 shuts off.  If you run the CO2 continuously (an many of us do)
and don't use a solenoid or controller, the only time you CO2 will shut off
is when the CO2 tank runs out of gas.  A two-gauge regulator will allow one
to monitor this situation as closely as one wants.

In my view there is simply no substitute for understanding the basic
relationship among CO2, pH and KH.

Regards, Steve Dixon in San Francisco