The Tank to bowl gasket: If you are seeing water on the floor in the general vicinity of the toilet, turn off the water as described earlier. If after you have turned off the water you find that water is still coming from the tank, but not necessarily all going to the bowl, you probably have a leak from the gasket or the bolts that connect the tank to the bowl. Again, if you are reading this in the middle of the night, shut off the water, drain the tank and mop up the mess and go back to bed, you’ll want to take care of this in the morning.
What you will need to complete this repair is a new gasket that matches the size of the hole in the tank and bowl (2 or 3 inch), and you’ll most likely want to get new bolts and washers. I suggest replacing these all at the same time because once you remove the tank from the bowl, getting old washers to be as water tight as they were before is difficult to do. Again, like with the flapper, signs that it is time to replace these rubber washers and gaskets are the wear and tear of them not being smooth around the edges. If they look like they have met up with a belt sander and they are leaking yet, they will soon. Also with the rubber washers, if when you touch them the water becomes black as if a squid had just released all the ink in its ink sac, it is a good time to replace them.
So with the water off, the tank emptied, your screw driver in hand (it will either be a flathead or a phillips head ( a slot - or a +) and either an adjustable wrench or if you have one a pass through socket wrench works best; loosen the bolts and the nut underneath. Once you have all the bolts free of the bowl, you can lift the tank off the toilet. I like to have a towel over the seat of the toilet and you can then place the tank on the flat side to work on it. The rubber gasket will peal away from the lock nut on the bottom. Now it is time to go get the parts if you haven’t already, or if you didn’t know that there would be a couple of different types of gaskets. Take the one you just pulled off with you so you can match the size.
NOTE: There are some gaskets are also attached to the bolts and have rubber that goes through the bolt holes and the bolts go through the rubber. You can find these types of gaskets at the hardware store or home centers, but it is not necessary to use the exact same type, the fix I am detailing here will still work, and in some cases will save you money on parts.
Now that you have the correct size gasket and the new bolts and washers to connect the tank and the toilet bowl together, you are ready to proceed. To clean off that disgusting crud that is all around the connection hole. Most of it is water residue… unless you have guys in the house who stand up… then it can be something else. Just saying.
Okay, now you have a nice clean and hopefully sterile toilet top, and you can put the gasket back on. Depending on the type of gasket you got, either you just slip it on over the lock nut, or you will have to line it up so the cut out inside matches up to the locking nut. Now you can put the bolts back into place. The tank should still be sitting on the seat, place one washer over the bolt and fit it all the way to the head of the bolt. Now place the bolt through the hole so that the head of the bolt is inside the tank, thread the second washer over the bolt. Repeat this for the other one or two bolts depending on how many your tank needs. Now you can place the tank back into position by guiding the bolts through the matching holes on the bowl. Now comes the fun part. Now with your screwdriver putting pressure down, place in this order onto the bolt: the third rubber washer, the metal washer and then the lock nut.
TIP: if you have the pass through socket wrench, put the nut in the wrench and then the washers so that the rubber washer is on top, and then smile to yourself as you think of all the other poor saps you will most likely be fumbling with these washers and nuts trying to get them on.
DON’T OVER TIGHTEN THE BOLTS. Sorry I didn’t mean to yell but it can be costly and cause injury if you over tighten the bolts and break the porcelain thrown. The best way I have found to tighten the bolts is to start with the back bolt, and get the washer to touch the toilet. Then move to one of the sides bolts (if you only have two bolts it is more like a see-saw action to tightening). Once you have all the washers touching the toilet, just kind of go around until you notice that they don’t move easily. Check the level of the tank both in the back of the tank and from the back to the front. You don’t need to be squishing the gaskets, they just need to be filling in the space and be snug. Once the tank is level and it doesn’t rock with a little pressure from you, it is time to turn on the water. I suggest turning it back on slowly, so that if there are any leaks, you don’t get a flood. If you see water coming out from the gasket, then you need to tighten all or a couple of the bolts. If you see water coming down one of the bolts, you’ll need to tighten that bolt. If you don’t see any water, then continue to open up the shut off valve. If you have the valve opened all the way and there is no water escaping it is time for the flush test.
Flush the toilet at least 5 times. This gives you a pretty good indication that the problem is solved. Congratulations!