Thanks Chem Geek!

Although that makes complete sense to me that the iron test would measure sequestered ions, what I'm not understanding (and please pardon my pool/chem ignorance) is how my iron tests went from 0.5 in the days following (6 quarts of) sequestrants. Then we shocked, turned back to brown, filtered to clear, then iron was down to 0. Then suddenly, iron went up to 5 (in 48 hrs - from 0) without adding so much as 1/2" of water. I would think for that to happen in a 13.5K gal pool, I would have seen sludge come out of the hose and I didn't, nor did I get any staining with that top-off.