I replaced the flapper--but why does my toilet still leak?

[HarleyKelvin]

Flapper in toilet has been replaced.
I called a plumber and he replaced it again. But the toilet keeps leaking.
When I say leak, it is NOT leaking on the floor, but somehow the water is leaking from the tank into the commode.
I called the plumber again and he came out and found that the bottom of the flapper was slimey, only after replacing the flapper after 1 month.
He seemed to think it had something to do with the water and chlorine.
Anyway, he came out again yesterday and replaced the flapper again and about 4 hours later, it was leaking again.

[nnnik]

Was it the same plumber every time?

[HarleyKelvin]

No, I hired the different plumbers but they couldn't resolve my problem. Now I can't understand what should I do?