yes, normally exhaust fans are to remove excessive heat.
could be an oxygen / CO2 thing as well, but that isn't usually an issue.

LEDs are a joke. They work, but you might as well use spiral florescents, which give you more light for about 1/10th the cost if you build them yourself.

Your tent is 4x4x6= 96 cubic feet. Your exhaust blower moves that much air in 24 seconds. You'd be fine with a tiny computer fan instead of your big blower. I would just add more light, which would make more heat, and then you could put that blower to use.

And you don't need both an intake and an exhaust fan. One of them can be passive, which means just a hole. Air pressure will make fresh air find a way inside the tent if you are actively exhausting.

For most people, if you have access to fresh air at all, CO2 will be a waste of time. That CO2 generating bag thing would be most useful if you had everything sealed up in a closet and were not able to ventilate with fresh air. But it's tricky, because you don't know the CO2 level of the air without a meter ($500 US last I checked), and too much is bad.

If I were you, I would lose the CO2 bag, add more light, and run a smaller blower constantly to regulate temperature. Stanley makes a variable speed floor blower and sells it at Wal-Mart for $40. I think the 3 cfm settings are 90,120, and 250. Also, 4" ducting would be plenty, rather than the 6".

Everything Cole Robbie said is 100% true. Well said.

Yup