PE: Thanks very much, I'll look for a kit with some Na2S. ( I only had a year of chemistry in college, so I'll just follow the directions! )
Our water is from our well, run through a calcite filter and then a water softener ( with KCl instead of sodium ) and finally a reverse osmosis filter. There aren't many dissolved solids after that, but our water is probably different from other places. Testing it will be!