Periodically you need to throw some copper into the septic system. Plants hate copper ions. I've heard tales of flushing a penny every month, but as little copper is in a penny nowdays I think buying some real copper mesh and tossing it down twice a year is a better solution.

http://www.amazon.com/Copper-Mesh-Rats-Birds-Control/dp/B0001IMLTO/ref=cm_cr_pr_product_top