From what I understand a single stage regulator will need to be adjusted up to maintain the same dosage (or bubble count) as the cylinder pressure decreases.
Not quite.
A fire extinguisher is full of liquid CO2 under about 55bar (800psi) pressure. As you use CO2, the pressure does not fall, it stays at 55bar (800psi) as the liquid evaporates keeping the pressure constant.
Eventually the liquid CO2 runs out leaving just gas at 55bar and this is when the pressure will start to drop and a single stage regulator may start to produce a lesser bubble rate.
However my single stage CO2 supermarket regulator starts lowering its bubble rate when pressure gets below 25-30bar (500psi) and I know the bottle has only a day or two left. My regulator eventually just stops delivering CO2 when bottle is very low (110-15bar ?).
Some "cheaper" single stage regulators can suffer from end of tank dump. This is where as the bottle pressure falls the regulator loses regulation and dumps the remaining bottle contents in one go. This is possibly OK when an "empty" 14gr aerosol sized container is dumped as b*gger all CO2 is dumped, but this is an issue when 100's of gr of CO2 is dumped when using a bigger CO2 bottle. People have wiped out their fish due to poor quality low cost regulators dumping CO2 when bottle is low.
Dual stage regulators supposedly don't suffer from this, but again cheap dual stage regulators have been reported to dump when bottle pressure is low.
Some cheaper single stage regulators often aren't actual regulators (see pick below), but really should be classed as flow control valves. They consist of either a metal sponge or metal plate with a pin hole in it and this is what controls the flow. They don't actually regulate the pressure, but produce a pressure drop across the sponge/pin hole restriction. They can't be used with solenoid valves as they require constant flow to drop the pressure.
