Replacing the seal between your toilet tank and the toilet bowl will stop the water leak between the two. The seal is a hard rubber ring that surrounds the flush valve retaining nut and fits inside the flush hole in the toilet bowl flange. It is a good idea to remove the old seal and take it to a hardware store. There are several different sizes and you want to be sure of having the correct size for your toilet.
Reach behind the toilet bowl on turn off the water supply valve. Lift the toilet tank lid of the toilet tank and set the lid on a flat surface out of the way. Flush the toilet and keep the flush handle down to allow as much water to drain from the tank as possible.
Place a large towel inside the toilet tank to soak up the remaining water. Wring the towel in a bucket and continue soaking the water out of the tank until there is no water left.
Twist the water supply hose connection counter-clockwise with a pair of channel lock pliers where the supply hose connects to the toilet tank fill valve. Locate the two bolts that secure the toilet tank to the toilet bowl.
Ask a friend to hold a large flathead screwdriver on the securing bolt head inside the toilet tank. Remove the nut below the toilet tank securing the bolt and the tank to the toilet bowl with a wrench. If the nut and bolt have corrosion and will not turn, carefully cut the bolt between the nut and the tank with a hacksaw.
Lift the toilet tank off the toilet bowl and set the tank upside down on a flat surface. Remove the defective toilet tank seal from the bottom of the tank where it surround the flush valve retaining nut.
Take the seal to a hardware store to purchase a replacement. Compare the new seal to the faulty seal for proper sizing. You should also purchase two more tank bolts with rubber washers.
Place the wide end of the toilet tank seal around the flush valve retaining nut. The taper end of the seal will be pointing away from the tank. Lift the tank up and position the tank over the toilet bowl flange. The seal will fit in the hole in the flange. Press down slightly to seat the seal. Do not push down hard, you only need to feel that the seal is making contact without wobbling.
Place the rubber washers around each of the new tank bolts and insert each bolt in the tank and through the toilet bowl flange. Thread the nuts hand tight onto each bolt. Ask a friend to hold the retaining nuts with the wrench while you tighten the bolts from inside the tank with the flathead screwdriver. Snug the bolts tight but do nut over tighten them causing the rubber washers to flare out around the bolt heads.
Wrap new plumbers tape around the exposed threads on the fill valve below the toilet tank. Reattach the water supply hose to the fill valve and turn on the water supply.
Allow the tank to fill and then flush the toilet one time. Check for leaks especially around the water supply connection. Place the toilet tank lid back on top of the tank.