If here is a black flapper type thing in about the middle of the tank, it probably needs replacement. If it is leaking water into the tank as you suspect, go buy one and install it. (You can put food coloring in the tank and see if it shows up in the bowl, as a check if youre not sure.) A Korky flapper will work. But if you can purchase a Kohler flapper, it will work better. It is an easy job.
