I have a slow leak in my toilet tank. Leaks down into the bowl, drips. Tank water level gets lower until it hits the low point and refills. I can see the water leaking from the bowl rim. Than said, I replaced the flapper valve with a new flapper. Still leaking. I replaced the entire overflow tube/drain fixture thinking it could be the rim or the seal between the overflow fixture and the tank. No luck still leaking. I removed the tank again and tightened down the plastic nut on the bottom of the tank that holds the overflow tube/fixture in place. No luck still leaks into the bowl. No external leaks. Any suggestions before I call for the costly option of getting a plumber?