A toilet tank should fill with water until the surface sits approximately 1 inch below the top rim of the central overflow tube (or exactly aligned with the manufacturer's water line mark stamped on the inside porcelain wall). This precise water height ensures there is enough hydrostatic volume and gravitational force to trigger a powerful, complete flush cycle while preventing water from spilling wastefully down the overflow tube into the bowl. Verification: Measure the distance from the water surface to the top of the overflow pipe with a ruler to confirm it is roughly 1 inch.
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