Dave:
If you check the archives you can see the cleaning process for your tank from Peggie's previous posts. I think it is something like 8oz of bleach/10 gal. of water. Be sure to let it get into all of the fixtures so the entire system is sanitized. Let it sit in your system for 8 - 24 hrs. Then flush the system with clean fresh water to remove all of the bleach.
I think it depends on what you use your water for. Do you drink it from the tap or are you making coffee and washing dishes. We go through about 75 gals every 3-4 weeks if we are not showing with it.