I have a very strong mildew/mold smell coming from my tub drain. The tub drains slowly, so it may be that something is clogging it and causing the smell. However, I have a Hydroblast septic system and would like to know if it’s safe to pour bleach down the drain. Additionally, what is safe to remove clogs from septic lines? Two things to consider are that 1) I have cast iron drains and 2) the plumbing in the house is 40 years old. Any advice is greatly appreciated!