There are two common ways a toilet can leak. The gasket (or flapper) at the bottom of the tank is the most common cause. However, if replacing it doesn't work, then you may have a leak in the fill valve, which will cause the water in the tank to rise above the overflow level and dump water into the overflow pipe. Sometimes, the fill valve will seem to work, but an hour or so after flushing, a slow leak in the fill valve will allow water to dump into the overflow. I would not put any chemicals in the toilet tank, only in the bowl.
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