A coolant tank was a containment unit to store gases or liquids, such as plasma coolant, used to cool parts of a starship or any vessel or base whose power generation units create extreme excess heat that could damage or destroy nearby components or structures.

The USS Enterprise-E had coolant tanks on the left and right sides of the warp core. One tank was punctured by Data in a final blow to the Borg Queen. (Star Trek: First Contact)