Well, your test runs tell you that neither copper nor manganese are the problem . . . which probably means iron is.

The metals bucket test is described in this forum section (Dealing with Stains & Metals, . . . and 'Minerals' & 'Ions') and allows you to test for staining metals in your water in a non-quantitative, but more reliable manner.

The 2004 MSDS I have for Alden Leeds Conquest reports that it *was* 60% HEDP, so I assume it probably still is. You might want to call A-L and ask, though. 1 gallon of 60% HEDP in 80,000 gallon is pretty high, and could cause problems with calcium > 300 ppm.

(At least for water, your lab is correct that 1mg / L = 1 ppm.)