Does silver plate out of your fixer onto the bottle?

I think is what's happening. I reuse my rapid fix, both film and paper. I mix Kodak RF per directions with hard tap water and no hardener. Sometimes, not always, when it stands, the plastic (milk) bottle turns black. It looks like silver plated to the inside of the bottle. The fix still works, but I dump it for fear of getting those little bits of silver stuck to an image somewhere.

Any body else have this issue? Is there a cure? Would distilled water help? My water is pretty hard with calcium, but not much else. No chlorine. Not much iron.

Thanks, EW

I take it that the jug has been in use a while, that happens to mine too.
I don't like milk jugs, I prefer used, green, 2 liter soda bottles.