An overfilled septic tank can cause backups in a sewer line, causing many problems in a Plumbing system. If a septic tank has overflowed, it is usually clogged with excessive organic debris. A septic tank cleaner can be a good first step when treating a septic tank overflow. Septic tank cleaners contain natural bacteria and enzymes, designed to break down organic material in human waste. Excessive disposal of bleach and detergents into a septic line can kill these bacteria, preventing the breakdown of septic waste in a tank. If a septic cleaner doesn't clear a tank after a week, it may be necessary to use a professional septic tank cleaning service.