What’s the best way to clean shower door hard water stains

vinegar, lime-a-way, clr, all work on lime scale dont use any thing abrasive on the shower

If the buildup is that dense then you need to take a straight edge razor blade and start in an inconspicuous area and gently at first run the blade down at an angle to remove the calcium deposit.

Keep wiping the blade off and do the entire glass once you get the hang of it. The trick is to a picture scratchingthe glass.

Once you've removed buildup then take a used dryer towel and dampen it. Wipe the glass down and get more used dryer towels dampened and continue.

Finally wipe the glass with water dampened microfiber towel. In order to keep your glass free of these buildups you need to squeegee glass and dry with towel after every use.

I know it's a hassle but it will save you the time and effort in future of using a blade.

Once you have restored glass you can purchase a water repellent spray to apply one time that you must get from an Auto supplies store (used for windshields).