Hi Paula: By fill tube, do you mean the cold water inlet line? If memory serves, that line cannot be PVC, but must be CPVC, copper or galvanized.....sounds like somebody boo-booed. The fault here is not the water heater, but the person who used PVC.
Or, do you mean the annode rod that actually goes down into the water heater(there is no water supply hook up to it)? This 'sacraficial rod' is supposed to corrode (usually made of zinc) rather than the inside of your water tank. All good water heaters (except those tanks made of pure copper, which I haven't seen for quite a while (are they still made?) ) have this annode rod. The trick here is to make sure that as a homeowner, you have that rod at least inspected every 3 to 4 years, depending on you local water's mineral content.
Oh, and a good flushing out of your plumbing pipes is probably a good thing to do every decade or so (smile)