Thank for responding. I guess about 10 years old. It not my house. I just remember something in the pass. I at my own home, one time left stool's cleaner fluid set for a long period of time (24 hours) and leave a ring in bowl. It may mean that the left over deposit at water level is dry up before I flush it down. Many years ago at difference home, I put bleach in stool and cause the coating to dull and checked.