Abstract: A supplemental fluid tank, preferably having two chambers each partially containing a fluid, fluid communicated intermediate a fuel tank and vent to reduce fuel vapor emissions, particularly for a boat. More specifically, when fuel is used or cooled, pressure or volume, respectively, of the remaining fuel in the fuel tank is reduced in prior art systems. Accordingly, air is drawn into the fuel tank through the vent line and becomes saturated with fuel (i.e., fuel vapor). Conversely, when fuel in the fuel tank is warmed it expands and fuel vapor is forced out of the vent into the environment. An exemplary embodiment reduces entry of air in through the vent and escape of fuel vapor out of the vent using two intermediate chambers in fluid communication with each other, each preferably having a non-evaporative fluid (e.g., oil), to provide volume/pressure compensation of the fuel in the fuel tank.