Spray some WD40 in the bowl and leave for a few minutes and use a toilet brush to remove stains. It works really well.
CLR is the easiest
Hard water is such a problem, isn't it? I use the disposable Clorox scrubbers (they attach to a handle).
A few squirts of Dawn dish liquid, pour in vinegar. Agitate with a toilet brush. Leave for an hour or two. Lightly scrub with a toilet brush. Suds suspend the vinegar above the water line.
It is really not that hard, but just a job none of us really like to tackle. Check out the information below, as well as the others above. Good luck to you!
I use the pumice stick I get at grocery store