The plastic water storage tank, usually black, mounted on a roof or a stand.
Large parts of Accra are on informal water rationing — the mains come at night, or on certain days, or during certain weeks. The polytank fills when they run, and you draw from it when they do not.
It is not optional equipment in most of the city. A property in a rationed area without storage is a property you will regret.
Before renting, establish: is there a tank, how big, is there a pump, who pays for the pump’s electricity, is the tank shared, and who cleans it. Tanks need cleaning.