We have lived in our 35 year old home for 1 year now and use septo-bac every Monday in our toilet. By Friday there is a sewage smell coming from the toilet so strong that we keep the door closed and the window open until Monday when we use another septo-bac package. Is this a sign that our septic system needs to be cleaned out?

I think it is more of a sign that the wax ring under the toilet isn't sealed. If it was sealed, there is no way gases would back up. Sit on the toilet and rock side to side. Is it loose? If so, CAREFULLY tighten the mounting bolts. Sniff around the base. Smell coming from there? If tightening doesn't work, you will need to remove the toilet and replace the old ring with, perhaps, a thicker one.

Yep I agree with Ma0641, if the wax ring fails you will have the smells, follow his instructions and you will be OK, Thanks.

John