Great topic! Cigarette smoke is actually quite dangerous to fish. Cigarettes contain up to 3% ammonia and nicotine is toxic fish. We are all aware of the dangers of smoking and the effects of second hand smoke so its common sense on this one.
The scented candles are a topic of debate as some do release carcinogens as well as lead and the FDA is considering a mandate on them due to the potential hazards they can pose. There are quite a few articles that cover the risks associated with burning scented candles (to humans) and its recommended to always use them in a well ventilated area or opt for organic/naturally sourced candles. I did a quick look and could not find any research at the moment as it specifically relates to aquariums and candles/incense but I do recall reading articles in respect to this before. I will see if I can dig them up later but if anyone else finds something of interest, please post it for us!