I've never bothered figuring it out since I just set myself a fish-related budget every month and stay within that range and stick it in savings if I don't use it all. I don't pay for water or electricity here so that doesn't account in.
What stuuk1 said on the lights/water, but you'd also have to account for buying filter media, bulbs, replacing broken stuff, decorations, food, livestock, medications, fertilizers, water conditioners, things you just "gotta have", etc. This part would be easier because you could just keep and add up any fish-related receipts and figure out averages for how much you spend a month/week.