My toilet tank fills between flushes. This usually happens because water is leaking by the flapper in the bottom of the tank causing the water level in the toilet tank to drop. When the water drops below a preset level the float on the fill valve drops causing the valve to open and the toilet tank to refill. Often times this can be remedied by replacing the flapper but the flush valve that the flapper seats against may have to be replaced as well. If this were the case then the tank would have to be removed from the bowl to make the repair. Depending on the age and condition of the toilet it may be more cost effective in the long run to just replace the toilet. Benjamin Franklin Plumbers are trained to make you the customer aware of all your options so you can chose the coarse of action that best fits your current situation.