I'm using a CO2 w/ regulator. Yes, DON'T use it when your lights are off. If the room is sealed off w/o exhaust fans, then you shouldn't overload the room w/ CO2. Too much is bad (and can be toxic to you buddy). I'm running mine with temps at 74-80F. You can operate them at higher temps which enables your plants to survive w/ the increased temps. I wouldn't worry about buying an air conditioner. Just look at your plants, like the above quote said, your temps at 88-90 are not too bad. If your plants are doing ok, don't worry about it. You are going to spend a lot of money for something you may not need. Peace my brother.