5000-Gallon Makeup Water Tank: Sizing, Dimensions And Coating For A Compact Buffer A 5000-gallon makeup water tank is a compact buffer - about 18.9 m3 - that tops up cooling towers, reverse-osmosis permeate, or small process loops where a large welded tank is oversized and slow to install. The risk is treating the gallon number as the whole spec and ignoring the diameter, coating, and skid details that decide whether the tank fits the pad and stays leak-free. This article is