black residue in fixer bottles

I was cleaning up in the darkroom since yesterday was our local haz-waste collection day. I tossed a couple of old plastic bottles that had been storing used fixer. I've noticed that black residue forms on the bottoms of these bottles. Is this silver settling out of solution or something else?

I've got a new bucket for collection and may try to implement some silver recovery.

Yes it's colloidal silver. Why not buy a Silver Magnet from jnanian.

Ian

residue

What does the Silver Magnet do? do u collect and sell the silver?