So I'm now down to 8 hours of CO2 per day with no algae. Is there a rough correlation between lighting duration, intensity, and CO2?

The tank is 75g, heavily planted with 6*54w HO T5 daylight bulbs. I decided to slowly cut down on the hours of full beam on the basis that the sun is not directly overhead for most of the day. I done similar with my reef.

For the last three months I have been running two bulbs for 11 hours and the full six bulbs for only 5 hours. Since the tank is stable I reckon I could cut down to four hours of full light and maybe 5 hours of CO2. Am I chancing it?