Another issue you may consider is better oxygen exchange via increased surface agitation.
If you think you have enough agitation, you don't.
Far better to skim the DOC's from the surface, but that's another story.
Light intensity drives plant growth. Lower the CO2, lower the PAR or PUR and you should be fine with no algae issue.
Light is also on a logarithmic scale, however plants can utilize significant amounts of light well in excess of the levels we can control in an aquarium. Since we are already far away from the light saturation point, reduction in light can cause a significant reduction in growth.
Where CO2 concentration is near saturation for growth, reducing CO2 concentration will only reduce growth by a small margin. Look at some graphs in the PDF I linked. As CO2 is increased above equilibrium with the atmosphere growth rate is increased exponentially. However, as CO2 concentration reaches saturation (actually quite a bit before saturation), growth levels tapper off.
Increase CO2 concentration from 3ppm to 20ppm and the growth rate will increase significantly. Increase CO2 concentration from 20ppm to 40ppm, and the growth rate only increases slightly. This allows some margin of error. So for those of us who like to push things to the limit, we can actually reduce CO2 concentration somewhat, suffer little reduction in growth rates of the aquarium, and make life more comfortable for other inhabitants.
Reducing both CO2 and light will reduce growth significantly, which is probably not the outcome those with excess CO2 supply desire.
Algae adapt to changes in CO2 concentration far quicker then plants are able to adapt. We don't set CO2 concentration to control algae, we set CO2 concentration for optimum plant growth, since it's healthy plant growth that reduces the ability of algae to thrive. Once we have our desired CO2 level, it's all about maintaining consistency. It's also important to remember that CO2 concentration has a direct relationship with carbonate levels, so to maintain consistent CO2 levels in the aquarium, pH alone is not a sufficient indicator (actually it's an ok indicator, but requires careful monitoring which rules out pH controlled CO2 injection), one must also monitor carbonate concentration. If you have some substrate that reduces carbonate concentration, then it is also reducing CO2 concentration for the same pH.
In this case, with a water change the aquarium has some specific carbonate level, over time the carbonate level is reduced, which also reduces CO2 concentration for the same pH
. Next week with another water change, carbonate levels are increased, which also increases CO2 concentration for the same pH
Bouncing CO2 concentrations for the same pH, which provides conditions more suitable for algae then plants. In this specific example, a carbonate source added to the aquarium (small amount of shell grit, or whatever), will offset carbonate loss and help to maintain consistent CO2 concentrations even during water changes.