Probably the shops measured 100 - 200 ppb, not ppm. If I'm correct, that's nothing to be concerned about at present.

You can do lime softening with soda ash (NaOH). This explains it: http://www.poolforum.com/pf2/showthread.php?16992

If you get your pH above 7.2, and keep trichlor out of the skimmer, your heater will not continue to erode.