For a heavy layer reduce the water level to the lowest point and use
about a
quart of muriatic acid with the window open. The fumes are nasty.

Be very careful if you use the acid!!!!!!!!!!!!!!!

Vinegar may also do it and it's obviously safer to use.

Vinegar is excellent for removing calcium spots on cars hit by
sprinklers with hard water.

Be thankful that you have hard water. Men who drink hard water get
HARD erections (unlike those who drink soft water and have to re-marry
every year).

Voice of experience?