John N. said:
I respectfully disagree with this. DIY can be highly variable especially during the beginning and ending of the yeast reaction. However, I do agree that excel can be used up at a variable rate also, but the fact that it's dosed every other day or daily makes up for that fact, where as with DIY CO2 one typically doesn't monitor for a daily constant output.
For small tanks under 10 gallons feel free to do Excel or DIY.
For tanks larger than 10 and between 20 gallons DIY CO2 is more cost effective and as efficient.
With your amount of light though be sure you are getting high levels of CO2, with DIY or excel, or you will likely encounter major algae problems.
-John N.