noun
tank
TANK/tæŋk/
A large container for holding liquids, gases, or other contents.
The open tank holds water in its cylindrical body.

Etymology
Portuguese tanque, “reservoir,” is one proposed source; another account traces the word to Indian reservoir terms. English extended the meaning from a pool for storing water to an artificial container.
In context
during Integrity tests Engineers will fill the tank with water and then pressurize it in order to test for leaks