When bleach is mixed with acids (such as those found in toilet bowl cleaners), it reacts with them to form chlorine
gas, which can actually be deadly. When it is mixed with
ammonia, it can create chloramine gas, another toxic substance. Short-term exposure to chloramine gas may cause mild asthmatic symptoms or more serious respiratory problems. It can even form a toxic gas when hyper-chlorinated by hot water.
In the environment, sodium hypochlorite is acutely toxic to fish.