A large container for storing water, often used in buildings, factories, or municipal water systems.
This building has a large water tank.
貯水槽 is a technical term specifically for a water storage tank, while タンク is a broader, everyday word covering many kinds of containers for liquid or gas.
貯水槽 stores water for later use, while 浄化槽 treats and purifies wastewater before it is discharged.
Compound of 貯水 (water storage) + 槽 (tank, vat).