Burning Candles?

Vaporized liquids can re-condense around cool things. If you burn a LOT of candles, there could be some small amount of candle junk getting into the tank, but I think it would take years for it to build up to any noticeable level, especially with regular water changes.

Now there are some things that I have heard of that can really mess your tank up. Keeping your tank in the basement or under and around a heater means that the CO2 in the air is higher and will mess with your PH. Also, pet potties (cat box, untrained puppy, etc.) put a lot of ammonia in the air that will also attack your tank. If you are cleaning your house, even if you are not spraying anything, be careful about what goes into the air. Anything that smells really strongly is releasing a lot of hydrocarbons into the air which can make it into your tank.