Replacing the toilet tank to bowl gasket because of leaks are rare, but toilet gaskets do dry out and crack over time that leads to water leakage on to the floor.
How to Replace Tank-to-Bowl Gasket
1a. Shut off the water.
Hold the flush handle down to empty the tank. Sponge out the remaining water.
1b. Remove the toilet tank.
Disconnect the water supply hose from the tank.
Unscrew the toilet tank-to-bowl bolt nuts.
Lift the tank off the bowl and place on a flat surface.
2. Remove the rubber gasket
Slide off rubber gasket and bring to the store to match a replacement.
3. Attach the new gasket
Slide on the new gasket assuring the tapered side facing away from the tank.
Move it around so it sits and seals correctly.
4. Replace the tank
Set the toilet tank onto the bowl.
Press down on the tank so the gasket seats properly.
5. Tighten the toilet bolts
Use new rubber tank washers.
Insert the rubber washers in to the tank, then the bolts through the washers.
Do not use metal washers inside the tank.
From under the bowl, hand-tighten the bolts with another washer and nut.
Tighten the toilet bolts alternately, first one side then the other until the tank doesn’t wobble.
The tank will still move some because the rubber gasket is not supportive or firm.
NOTE: Don’t over-tighten the toilet tank bolts to crack the porcelain.
6. Reconnect the water supply hose to the fill valve Flush and check for leaks.