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If you determined in the previous step that the water level is rising above the top of the flush valve overflow tube, the fill valve's height may be incorrect. First, confirm that the waterline mark on your fill valve is set to the same height as the toilet's manufacturer's water line. If the manufacturer's waterline isn't visible, set the waterline on the fill valve 1/2 inch below the top of the flush valve overflow tube. If the valve is currently set too high, turn off the water supply to the toilet (usually a small valve near the base of the toilet on the wall). Flush the toilet to empty the tank. If your fill valve has a tamperproof key (like the Korky QuietFill valve), remove it. Adjust the height of the fill valve by twisting the top half of the valve counter-clockwise, pushing it down until the correct water level is set, and then turning it clockwise until it snaps into the locked position. Finally, reinstall the tamperproof key if it was present, and turn the water supply back on.