alk: 85
pH: 7.4
Calcium: 220

The older stains are caused by running my pool at a higher pH, and probably overchlorinating. This is something I've since corrected. However, recently (2 months ago) someone dropped a metal nut into the pool, which made half a dozen penny-like stains on the bottom.
Since then, light tan stains have been appearing all over the pool. An ascorbic acid treatment along with a sequesterant did nothing for them. I followed the protocol on this site exactly, hoping that it might work for me.

Would dropping the pH to 7.0 damage the heater? There is another site on the web that advocates dropping the pH to 6.0 for a day or so, but that seems to have the potential to cause too much damage.