To fix a leak underneath a toilet tank, first shut off the water supply valve, flush to drain the tank completely, and disconnect the supply line. If the leak comes from the tank-to-bowl bolts, unbolt them and replace the rubber washers and brass bolts. If the leak comes from the large center flush valve nut, tighten the locking nut carefully or replace the tank-to-bowl gasket. Callout type="warning" title="Hygiene Warning": Wear rubber gloves when handling plumbing seals. Verification: Refill the tank and test with multiple flush cycles to ensure zero water leakage underneath.
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