Hey I also found that oilfield water chemistry places can hook you up with dissolved O2 test kits that read 0-100ppm of dissolved O2 for pretty cheap.
That's kinda weird, O2 is saturated in fresh, cold water at about 14 ppm. In salt water it is about half of that. I know which kits (HACH) you are talking about and don't recall what the range on them was. We were usually looking at O2 levels at less than 1 ppm in oilfield waters. We sold products (sulfite and bisulfite) that are chemically very similar to the dechlor compound (thiosulfate) we use in aquariums. I have an aquarium O2 test kit I bought years ago when I was having some problems with my first tank. Its not very accurate and its almost impossible to get a representative sample from an open system like an aquarium.

I wish I still had access to one of those Hach water analysis kits. They are so much better than even the best aquarium test kits, although I know that some of the reagents in the Seachem kits are made by Hach. The little foil packages are a Hach trademark.

I know all about H2S corrosion. I started off working on bactericides for sulfate reducing bacteria in oilfield waters in the early 80's. They are what cause the heavy pitting in steel that you probably see a lot of in the Permian Basin. They are similar to the bacteria that produce H2S in DSB in aquariums. I do think a lot the H2S people smell in a DSB is really organic mercaptans (the stuff they use to odorize natural gas with) that is a natural result of biological decay of proteins. I have seen what looks like Iron Sulfide in my own DSB when I moved it, but that could be iron salts of mercaptans as well.

OK, I'm probably way over a lot of people's heads so I'll shut up now.

Gary