I would like to model a plastic fish tank partially filled with water and having a heater in the bottom of the tank. There is a cover on the tank, but there's an airspace between the water & cover. I'd like to have convection occuring in the water and also in the air above the water and on the outside surfaces of the tank. Is it best to make a model of the water and assemble it to the tank model? Can this be done with a fluid subdomain by placing a lid at the top of the water that has an ambient pressure boundary condition?