Hard water stains, they are fucking hard as hell to clean. That green shit gets it off, but I forget what it's called. Miracle Green or sometjhing? I have gotten it off with a rotation of vinegar, acetone, and isopropyl. Use distilled water, and it won't happen as bad.
Be careful with salt man.that shit is abrasive as hell on glass. Vinegar and baking soda to make a paste and use some elbow grease. That'll clean it up without scratching.
I have not yet. I keep forgetting to get some when I'm at the store. Like the guy above me said I do not like using Epsom salt unless I absolutely have to (usually just 91% ISO for me) so I've been intrigued to see how well the simple green works, people say it's awesome.
CLR will make that white scale deposit disappear instantly. I'm sure it's safe to use on bongs because people use it to clean out their water faucets.