Using too much bleach kills the bacteria in the sewage works which break down the effluent.
And don't ever use bleach and other toilet cleaners at the same time. If bleach is mixed with ammonia, toxic gases called choloramine are created, which can cause coughing, wheezing, nausea, or watery eyes; or at higher concentrations lead to chest pain, wheezing, or pneumonia. Using it in tandem with certain toilet bowl cleaners, drain cleaners, and even plain old vinegar is no better: the combination of chlorine bleach and acid gives off a toxic chlorine gas that can cause burning eyes and breathing problems in small amounts, and be fatal at high levels.