Recently, after having let my 6 gal. glass carboy soak overnight in bleach-water, I found quite a lot of sediment at the bottom (and a good deal on the sides). A friend of mine suggested that it was hard-water calcification.
I have been unable to remove this sediment. I have used 1-Step, Iodophor, warm water, all with my carboy brush, and it seems to have little or no effect. I am currently trying phosphoric acid.