there is a third caegory. Inverted bottles methode. Basically it is a bottle in the tank upside down filled with CO2 gas.CO2 reactors and diffusers are the 2 categories.
It is much more efficient then diffusers since all the CO2 must dissolve into the water in order to get out. In my small aquarium with a diffuser my small paiintballl bottle lasted about 3 months. However with the inverted bottle methode my bottle lasts about 1 year.
And it will increase CO2 concentration in the tank to the point were CO2 starts outgassing from the water. Which is about 400ppm. Note Drop checkers can only detect CO2 at concentrations above 400ppm. So drop checkers won't work. PH probes are also not useful for meashing the PH effect of CO2 because carbonates and other minerals have a much larger effect on PH than CO2 (unless you use a lotto CO2).
Fortunately with this methode it is impossible to kill fish with too much CO2. CO2 is removed from the bottle as needed. if you put too much in it bubbles too the surface and dissipates into the air.
In the video the bottle is filled manually about once a day. I use a timer and carefully set my CO2 flow so that is just enough to fill the bottle in one minute..
And if cost was no option I would use 2 regulators on the CO2 bottle. one a general purpose capable of handling the full pressure in the bottle. Then use a second regulator optimize for low follow pressure output. I salvaged a ParkerQRM30 stainless steel zero to 30PSI regulator from scrap hardware. A Low flow and low pressure regulator .It cannot handle the full pressure in theCO2 bottle. but it has very good control on setting the output pressure. My Nilocg.com paint bottle regulator stays at 10psi and the needle valve is locked at it's minimum setting. I now adjust the Parker pressure regulator instead of the needle valve.