It depends on your water's mineral content and what your stains consist of to some degree.
I use a homemade recipe:
1 cup washing soda, in about 2 cups almost-boiling water, and stirred until *almost* dissolved (it never seems to *totally* dissolve)
add to tank
add 1 cup hydrogen peroxide to tank
fill tank with hot water.
Do *not* pre-mix this stuff or leave a full tank of it overnight. It expands! <LOL!> (don't ask how I know that <heh>)
You can also spot-treat *really* nasty stuff with a 1:1:1 ratio of washing soda, hydrogen peroxide and water, but be sure to dissolve the washing soda in the water first.
I still have to break out the hand-tool occasionally, but this takes care of *almost* everything stain-wise.... now if something is *stuck* on there, or if there's a greasy/fatty substance involved (I am never buying go-gurt again, it turns to *plastic*) vinegar sometimes works a lot better, but I don't like vinegar as well for overall cleaning.