The Portable Tank is a storage tank that can hold liquids. It has a capacity of 14 buckets (14000 mB) of fluid. Shift-right-clicking the tank with a Configurator will set it to auto-output, as indicated by the output and input ports (on the top and bottom) turning green.
By changing the mode of the Tank to Bucket Mode using the mode switch key (defaults to m) while sneaking, the tank functions as an high volume bucket. Right-clicking on a liquid will fill the tank, while shift-right-clicking will empty it.