Perimeter Stain

[waterbear]

CHEMISTY NERD STUFF ALERT! NOT FOR YUPPIES OR THE WEAK OF HEART!

I thought they would be filtered out too, but I am told that they do not get filtered out - they just stay in suspension.
Right Marie!, They are still in the water but in a 'chelated' form that keeps them from reacting with other chemicals like the chlorine or falling out of solution as stains. That is why it is important to keep up the maintenance doses of sequesterants if you have a metal problem.
But I have had my water tested after the treatments and they test as no metals, so I don't really know.
The metal ions are "seqeustered" (from dictionary.com)
se·ques·ter
v. se·ques·tered, se·ques·ter·ing, se·ques·ters
v. tr.

1. To cause to withdraw into seclusion.
2. To remove or set apart; segregate. See Synonyms at isolate.

The metal ions are, in effect, 'set apart' or inactivated by a process called 'chelation'

once again from dictonary.com
Chemistry. To combine (a metal ion) with a chemical compound to form a ring.
This ring structure means that there are no reactive points that can be affected by other chemicals in the water as long as the metal stays in this ring structure.
This is an oversimplification but you really do have a good understanding of the process and are the 'metal guru' of the forum!
Practical knowledge is MUCH MORE useful than the theoretical and you have that down!
I do know that if I keep my water treated with the sequestering agent I don't get the stains back. I keep reading all I can about the metals in swimming pools, but there doesn't seem to be a way to filter the metals out, except by doing it a way that Pool Doc says - but it involves careful control of ph and bleach to get the metals to fall out on the filter.
You are causing the metals to drop out of the ring structure and then fall out as stain on the filter by carefully controlling the parameters that break the chelation of the metal in a controlled way. It is tricky and doesn't always work right. PoolDoc understands this process much better than I do!
I don't have a complete understanding of it yet. If anyone out there can chime in and give a better answer, please do!

Hope I explained it a bit!
As a side note that might help, I have used copper in fish tanks to treat certain parasites. Copper is available as chelated or not. If you use chelated copper it doesn't show up on a test until it breaks away from the chelation. I have seen tanks that all of a sudden would test at high levels of copper a month or more after treating with chelated copper and showed practically no copper from the time of treatment until then. If the treatment with the chelated copper is overdosed it can prove deadly to some of the tank livestock when it becomes active and the copper levels shoot up and overdosing is easy because the copper doesn't show up in full concentration on testing in chelated form. I know that many copper based swimming pool algecides use chelated copper which WILL eventually cause staining down the line for the same reason.