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Every time a fish kill due to CO2 occurs, it almost always involves the
glass/ceramic diffusers. I think there is a fundamental flaw in these
system's design caused by a technology mismatch. I doubt that the diffuser
manufacturer and the regulator manufacturer foresaw these two products being
used together the way aquarium people are using them. The $65 single stage
regulators that we use are easily thrown out of balance by an open ended
outlet on the low pressure side, such as is caused when the diffusers
suddenly open up.
These inexpensive regulators depend on -some- back pressure on the low
pressure side, such as is provided by a needle valve, to keep the mechanism
in the regulator working properly. For a needle valve to work as a
safeguard, it has to be set so that the needle valve is setting the bubble
rate. Which means, for our purposes, that the needle valve is barely open.
If only two bubbles per second are making it past the needle valve, then
that is the most CO2 that can enter the water.
If a diffuser design demands that the needle valve be opened up more than
that to supply adequate pressure to the diffuser, then that diffuser is
inappropriate for aquarium use. The alternative is either a higher grade
regulator (more $) or a flowmeter (even more $) if you insist on using a
diffuser without a controlling needle valve. Economics and safety say use a
needle valve, ditch the diffuser, and just stick the CO2 line into a pump
intake. IMO, YMMV, ETC.