Connect the water supply line to the toilet tank by hand until you feel firm resistance, then secure with one additional quarter to half turn using pliers.

Start by confirming the shutoff valve is fully closed and the tank is empty of water. Inspect the supply line and the tank inlet for a compression gasket or rubber washer and replace it if it is flattened, cracked, or missing because a compromised gasket is the most common leak source. Thread the nut onto the tank inlet by hand, keeping the line straight so the ferrule seats evenly; hand-tighten until you feel firm resistance and the nut will no longer spin freely. Use adjustable pliers to turn the nut a further quarter to half turn only, avoiding overtightening which can crush the ferrule, strip threads, or crack the tank fitting. After restoring water pressure slowly, open the shutoff valve and let the tank fill while watching the connection for any small drips at the nut and at the valve end. Finally, flush the toilet several times and recheck both ends of the supply line for leaks; if you see a slow drip, tighten in very small increments and replace the gasket or the entire supply line assembly if tightening does not stop the leak.

The most common mistake is overtightening the nut until the ferrule or tank fitting is damaged which you can avoid by limiting tool use to a small additional turn and by replacing a mangled ferrule rather than trying to clamp harder. Another frequent issue is installing without a proper gasket or with a worn washer so always inspect and swap in a new rubber washer or compression sleeve prior to connection. A third concern is misaligned or kinked supply lines that stress the fitting and cause leaks, so route the line in a smooth curve, keep it straight into the inlet, and use a flexible braided hose if space is tight.

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