Ty D Bol is safe for toilets as well as septic systems.
I've used Ty D Bol in my toilet for 15 years now and it has never caused any issues and works wonders at cleaning the toilet and keeping it clean and fresh.
You can also use vinegar and baking soda as a natural toilet bowl cleaner which is also safe for toilets and safe for septic systems and sewer systems.
Ty D Bol is also septic safe and sewer system safe.
I also find that Dollar Generals brand of toilet bowl cleaner works wonders and works better than a lot of higher priced toilet bowl cleaners.