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re: CO2 regulators

I use a Blueline (Champion Lighting house brand) and a Knop CO2 regulator.  
They are two gauge, meaning they have a tank pressure gauge and a step down 
pressure gauge, which I generally keep adjusted to 5 psi or so. Each also has 
a needle valve for fine flow rate control. The step down pressure is 
adjustable from 0-15 psi on one and 0-30 psi on the other. Each has a 
solenoid that when energized or not acts as a simple on/off switch. The 
solenoid is controlled by a Ph Controller. There have been comments that 
these are unnecessary and that the pH probe can fail and flood the tank with 
CO2, killing the fish. They may not be necesary, but I have found them to 
work very well and maintain the pH at 6.7-6.9 with only probe calibration 
needed every 2-3 months. Also, IME, a pH probe will not suddenly "fail." What 
I see initially are slightly abnormal readings brought about by a small drift 
in the probe accuracy, and ending with a probe's inability to maintain 
constant readings after a calibration. This happens over time and I have 
always been able to tell far in advance when a probe was going to need 
replacing. It is just normal maintenance to check the regulator gauge 
readings, the bubble counter, and the Ph readings on your meter regularly 
(every 2-3 days for me).  

A properly set up step down valve and needle valve on the regulator will not 
flood the tank with excess CO2. It is not possible because the physical 
opening you set in the needle valve only lets so much thru. The solution is 
to set the flow rate to where this is not an issue. An In-line bubble counter 
allows you to see exactly how much is going into your tank. Mine are set at 
about 1-2 per second. Even given the slight chance that the probe would 
suddenly fail and give abnormally high pH readings to the controller, which 
is the only situation where the controller would turn the solenoid on, as 
abnormally low readings would shut it off, the regulator is only set to 1-2 
bubbles per second, which is not going to kill the fish.

When my tanks were initially set up I injected CO2 at the described bubble 
rate continuously for 2-3 months and never lost a fish because there was too 
much CO2 in the water. The controllers were added later.