In more understandable terms:
If the tank is surrounded by air, air and tank will exchange heat, air will get warmer, tank will get cooler. This exchange is influenced by several factors (surface, temperature difference, "wind") and by the "thermal mass" of both air and tank.
If the tank is immersed in water, and water is contained in the plastic basin, and the plastic basin is immersed in air the heat exchange will take place between the basin and air, and between the water surface and air.
But considering that the plastic basin, and water, have a considerably higher "thermal mass" than air, you will need "much more air" to cool down the higher thermal mass. That's why I expect the plastic basin with warm water to cool down more slowly than the tank when exposed to air.
In a word: you can use water as a accumulator of temperature that will discharge more slowly than air, when in contact with air. It will also "insulate" the tank from the cooler ambient air, a bit of a "thermal mass overcoat" for the tank.
(Not that I am sure of this. I am just reasoning).