So I just started using some new products from Titan Labs in my window cleaning business. I use the Glass Gleam 4 which is a professional window cleaning concentrate made for hard water areas. They make about 12 different products ranging from hard water stain removers to solar panel cleaning solution to Multi-Purpose Cleaner. If you are a (do-it-your-selfer) I would definitely recommend these products.
Check out their website because I don't think my explanations of the products would do them justice.
I do have a couple videos on my YouTube channel of me using GG4 and a product called Oil-Flo if you would like to check them out here are the links: