As mentioned in 'Toilets', septic tanks rely on bacteria to break down the wastes. A cocktail of harmful chemicals coming down the drain is going to result in a dead and smelly septic tank. A cup of bleach down the toilet will kill the smell for a day, but the smell will return as bad if not worse.
Instead, using the Odour Control Spray, at every flush will gradually bring the tank back to life. In severe cases a complete pumpout and restart may be necessary.
Add our Starter Culture treatment to inoculate the newly filled tank with 'good' bacteria to get the system off to a good start. It still means in future that you have to be careful of what goes down the drain, bleaches, disinfectants and antibiotics, kill all bacteria, not just the 'bad' guys.