Philip Ash wrote:

I used two of the hagen kits on my 75UK gal tank and noticed a big
improvement in my plant growth.
One made a noticable change, hence i bought a second. It may be a lower cost
way to experiment.

The hagen unit is rated for a 20 gallon tank. Yes, two will add CO2, but if you
are targeting 20ppm, then you need at least four of them.

Yeast based systems are not really practical for tanks over 40 gallons.