A "fire hydrant tank" might be referring to a couple of different things depending on the context, but typically, it could mean:
The water source for a fire hydrant: This is the tank or water supply system that feeds water into the fire hydrant when it's used in an emergency. These systems are usually part of a city's water infrastructure, and the water comes from a larger municipal reservoir or water system.
A tank that connects to a fire hydrant for specific purposes: In some places, especially for large-scale operations or for buildings with a dedicated fire suppression system, a fire hydrant might be connected to a tank that stores extra water to ensure that there is enough water pressure and supply when the hydrant is used.
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