Thank you. I was actually approaching this from the opposite perspective. I'm running a 65 gallon with moderate PAR (I think) and no CO2. I'm switching tanks to one more conducive to plant growth (shorter), though I'd actually rather keep it low-tech for the near future.
So is there a point at which my PAR will become too much and it will prohibit plant health and growth? At what point does PAR become 'too much' without an added CO2 supply?
The quick answer is when you have so much light the algae starts growing better than the plants.
That is why we add co2, so the plants grow faster and limit the alga's ability to grow. In almost every situation involving a non-co2 setup, an aquatic plant's growth will be limited by the amount of co2 it has access to, not the light.
Too many variables to calculate any sort of reasonable point where this kicks in.
Plant type, plant growth, amount of plants, bioload, substrate, fertilization (substrate or otherwise), light intensity, light duration, water chemistry...etc. All will play a role in this calculation.
It has been my experience that if you are wondering if you have to much light, you probably do. When I had my 90 gallon setup it was a stable low light setup. Things where growing and I had minimal algae. I added Co2 to this tank and the plants went insane (same low light). This tells me that co2 was my limiting factor, not the light. For grins, after I had co2, I doubled the lights. My plants didn't grow much faster, but my algae did. Not sure if that really answers your question...I guess my point is that lack of co2 is most likely already your limiting factor...adding more light isn't likely to speed up anything, but problems.