The water line is normally about 1/2" down from the overflow
If the flapper is leaking, the water level will drop, and then refill.
If that is the case
seal between flapper and flushvalve
seal between flushvalve and tank
If the tank overfills
The fill valve needs adjusting (Not normally ever done)
Or the fill valve is not shutting off.
Some fill valves look like this
I did get a call Saturday morning, perhaps about this.
I was in the middle of the walk at Seward Park.
Saturday May 30th Video Promo