I had to comment, some of the comments are nuts:
Candles are awesome. one parrafin candle produces 11 grams of co2 per hour. If you burn 4, that is over 1 lb of co2 per 12 hour bloom day, and 1.5 lbs of co2 per 18 hour veg day. Some of the comments on the safety are funny. No, don't put them in a tent, but plenty of gardens are able to accommodate 4 candles. If really worried about safety, place/float candles in water... There you go, you've just added 1 lb to 1.5 lbs of co2 to your garden per day... and an extra 50 lumens