We don't have a heater for the pool.

The only potential sources of metal I can think of are the salt and/or the city water used to occasionally top off the pool (I don't have to do that much since we usually get enough rain throughout the season).

What puzzles me is the fact that the bucket test shows no signs of metals present in the pool. What also is puzzling is that stains only start to appear when running the SWCG (Aqua Rite with T15 cell). I understand high chlorine levels can cause metals to fall out of suspension and attach to surfaces, but I've never noticed the start of any staining when shocking with just plain bleach (levels up to 30ppm+, with CYA level at 80ppm or less).

You mentioned looking at sequestering agents. I thought that's exactly what products like Jack's Magic are; ie they hold metals in suspension, preventing them from attaching to the pool surfaces. Am I misunderstanding this?

I've considered trying the CuLator, but from what I've read here it's still hadn't been proven to be effective. Also, it seems that it's slow to work (if it does work) requiring the use of a product like Jack's Magic to prevent staining in the meantime. And if metals are coming from our city water, then I would have to continue to use Jack's Magic until the CuLator removes the added metals.

This sounds like a pain to keep up with (having to constantly monitor for stains to determine when I can stop adding Jack's Magic and cleaning skimmer sockets to prevent the CuLator from gumming up). It also adds expense having to use both products (CuLator is not cheap).

Right now I just add a small weekly maintenance dose of Jack's Magic and don't have to worry about stains. Ideal? No. But it works for my pool.

I'm certainly open to ideas that would work and save money, but I'm not willing to risk having to deal with a stained pool again. I went through that once years ago, and once was enough (a PITA and expensive!).