you can raise co2 levels with baking soda and vinegar, or course you would have to do that multiple times a day, but at least it is something. also, if you put a candle INSIDE an empty coffee can (for protection) the candle using up oxygen and lets out co2. make sure you are around your grow area when doing this and that the candle in the can is in a safe place. and breathing and sighing on your plants when checking them out always helps.

i would mention sugar/water/yeast as making co2 also, but the yeast is VERY bad idea especially when working around hydroponic as those yeasts love water to multiply and you system could get a yeast infection.