Excellent question. I have heard not use window cleaner on the outsides of the tanks because the spray could get in the tank. I admit I spray window cleaner on a cloth away from the tanks and then clean the smudges off the outside. I don't think scented candles would be concentrated enough to dissolve into the water, but I would love to hear from so done who know.
I am careful when I use Febreeze not to spray near the tanks.