3. Lemon Kool-aid
If you are sick of Comet not working, this might do just the trick.

Lemon Kool-aid is the 25 cent cleaner! It works because the citric acid breaks down the hard water deposits with very little scrubbing.
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To clean your toilet bowl using Lemon Kool-Aid, flush the toilet, wait for the water to settle, and then sprinkle the Kool-Aid around the edges. Let it sit for about an hour and then use a toilet brush to scour the sides.