There's a min amount of light required.
Once you hit that, then adding CO2 is helpful.
Adding more light simply increases CO2 demand, more nutrient demand, thus you have more growth at progressively higher light intensities.
This is a trade off.
Most aquarist want good to moderate growth of any species they chose to grow, not the highest maximum yield. That's too much work and such systems are harder to maintain.
So we accept slower growth rates and easier management.
SuperColey1is pretty well astute on this topic
Spread, CO2, nutrients and other issues are part of this.
We are light limited in terms of growth for most tanks, a few might be PO4, NO3 or CO2 limited.
Still, since light is the largest energy input for most, it makes the most sense to limit it to control growth rates. It also makes the most sense to maximize the spread, the PAR/PUR, and of course our own aesthetics for color.
As lighting is the most stable parameter we can add and control in an aquarium and drives everything else, it should be the best choice of managing the system, not limiting PO4, even non CO2 systems are more dominately light limited than they are CO2 limited.
I think our color perception plays the most role in opinions about light, not actual rates of growth. I know of no planted aquarist other than myself that does dry weigh analysis, Tropica and Ole and few others do, but they are in the trade business.
So it's rather difficult for most aquarist to even compare other than their own sense of color, which is............mostly what they care about anyway, so that is often enough to support the argument
I cannot argue with that
I can compare dry weights between light sources however.
I can compare amounts of red pigment etc.
Regards,
Tom Barr