b. Yes, flush repeatadly, then after 3-4 flushes, bleach is a fine idea. A 1/4 cup per 10 gallons is a good starting point.
UNLESS you have stainless tanks -- bleach and regular stainless are not a good combo; it can really initiate/acccelerate corrosion, particularly at the welds. This is the case when dealing with welded stainless that is NOT low carbon (i.e., 316L.)