Of late, I've been dropping back from my ~40ppm area, not least of the reasons being that I now run it 24/7. However, my primary reason is to target what I believe is the pH 'sweet spot' for nutrient uptake by most plants. This seems to be in the pH 5.8-6.5 area. So, I watch my KH levels and adjust my CO2 accordingly. My plants grew fine at lower pH levels, but i'm in the fine-tuning stages of a year-long shift in overall strategy and don't believe the extra CO2 is helping anything..