Yes, Bar Keepers Friend can be used for some hard water stains. cleansers with grit can be used on glass, and they can scratch glass. There are a myriad of products you can use for water stains.
Remember, not all windows are made of glass...let alone the same type of glass. Not all stains are the same. The water in Florida is different PPM solids then in Canada, Kansas or Wyoming.
I have 4 different products I carry to use on different types and severity of stains ranging from simple cleaners to toxic materials like 550. Study, study and study the different products out there and pick the ones that work the best for you. What works for me might be too much for what you need and then you are repairing/replacing windows and frames