I built a CO2 system and used it for a couple of years. I used my home automation
to time the injection. In fact I still have a 20lb CO2 bottle that is empty.
In my experience it didn't work out too well. I live in AZ and it gets hot here.
The CO2 bottles are under very high pressure and if the bottle gets too hot
the excessive pressure valve trips and dumps the whole cylinder(this happened several
times). The gaskets that are between the regulator and the cylinder are plastic
and when they get hot they soften and then the CO2 leaks(lost a couple of bottles
of CO2 this way). Ended up being a expensive pain so I disconnected it.
My experience with CO2 was this, it did bring down the pH rather quickly,
but it was gone just as quickly. It was hard to keep a consistent pH during the
day. I think it would work fine in a commercial pool where the pump ran 24 hours
and the CO2 could be injected continuously. A home pool this really doesn't work
because the filter pump is only run about 6 hours at night(cheaper electric rates).
I plumbed the system with copper tubing and it seemed to hold up fine. I injected the
CO2 into my floor cleaning system so the CO2 was injected into the bottom of the pool.
The 20lb bottle would only last about 3 weeks in the summer(if it didn't leak or vent). This
made the operating cost twice as much as just using acid.
In a previous house I used a peristaltic pump to add acid to the pool and this system
seemed to be easier to use and trouble free. I used a 15 gal barrel and diluted the acid
3 to 1, which made it 10%. One of my future projects is to install a peristaltic pump
to add acid to this pool. With a 15 gal full barrel and a 3 to 1 dilution the barrel will last about
6 to 8 weeks.
Anyway this was my experience with CO2.