There are three possible causes of tank leaks.
Wear of rubber seal. If you notice any damage, it is recommended to replace it. Sometimes cleaning it from the plate is enough.
Float error. Normally, the water in the toilet should always be below the level of this hole. You can buy a new float or close the gap in it with something. Also, the problem may be that the float mount is broken. To do this, you will need to buy fasteners and sometimes a new float.
Drain valve failureIf the problem is there, you will need to replace the entire mechanism at once.
Source: Ferra

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