The most dangerous cleaning products you can use on your toilet if you have a septic system are those that contain chlorine bleach. This chemical is toxic to many living things, including bacteria. The bleach causes the proteins in the bacteria to lose their ability to grow. These proteins eventually clump together, killing the organism. Many people use bleach to clean the toilet bowl, but it's worth considering if you have a septic system. Of course, you should also avoid using antibacterial cleaning products. After all, the damage they can do to bacteria is in the name.
Hydrochloric acid is another common toilet cleaner that works well but can also kill bacteria. This is a strong acid that effectively eliminates the types of microorganisms you want to get rid of in the bathroom. However, these products cannot distinguish between the harmful bacteria you have in the toilet and helpful microorganisms that may be present in your septic tank. Check the ingredient list on the product label and any warnings. If a product is not suitable for wastewater systems, this can be mentioned.