This leak is usually caused by old rubber washers on the bolts that hold the tank to the bowl. Buy new ones. If they come with little metal washers that go on both sides of the rubbers beware! To fix leakage replace the bolts, you will only use one metal washer and two rubber washers to each bolt. This means you will need two brass (important) bolts, two nuts, two metal washers and four rubber washers. The metal washers go on the bottom just before the nuts. This is very important! If you use the metal washers inside the tank you will surely have a leak, if not today, very soon. Check here for an exploded view of a toilet, with a description and location of bolts. When tightening these bolts, go no further than drawing the tank and bowl together by tightening each bolt a little at a time and then switching to the other one. When they are just about tight enough you should be switching back and forth every quarter turn.