It depends on environmental conditions around your tanks and the climate. The bigger the temperature changes in the tank, the more air is pushed out by heated fuel and more pulled back in as the tank cools. Air coming into the tank brings moisture. In the winter or summer with little change in tank temps brings little new air whatever the amount of fuel. But if you're using your engines, the fuel is being heated because of the fuel returned to the tanks. My Detroit mains pump about 70 gallons an hour but return all but 8. If my day tank is kept full, there is little room for changing air even if the fuel heats. One reason tugs and ships use a day tank and keep their larger tanks cool and stable if they're not located where the engine room can heat them.
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