If the light fixture can be suspended above the tank, raising it a few inches will reduce the light intensity and make CO2 less critical. If you use a powerhead in-tank reactor, which it seems that you do, you can turn the powerhead on and off with the lights, which effectively turns off the CO2 when the lights go off. That lets you run higher concentration of CO2 in the water, without harmiing the fish at night. Then, using 3 - two liter bottles, setting them up one week apart, and changing one per week, will give you pretty uniform CO2 concentration from day to day. If it gives too much CO2, you can direct that powerhead's flow so it causes more surface ripple, and that both reduces the CO2 in the water, and adds more oxygen to the water, helping the fish tolerate the high CO2.