Hard-water stains are unsightly, and they are difficult to remove. Regular toilet bowl cleaners are often ineffective, although some cleaners are formulated to specifically treat hard-water stains. Even so, these are not always effective unless accompanied by heavy-duty scrubbing. 
The easiest way to eliminate hard water stains in a toilet bowl is to use a paste made of baking soda and vinegar. 
- Begin by closing the shutoff valve and flushing the toilet to get rid of the water in the bowl.
- Place one or two cups of baking soda in a container, and add enough vinegar to make a paste.
- Don a pair of rubber gloves, and apply the paste to the hard-water stains with your gloved hands.
- Allow the paste to sit for at least 10 minutes, then scrub the toilet bowl with a toilet brush or a wet pumice stone.
- Open the shutoff valve, and flush the toilet a couple of times to rinse.
Once you have removed the stains, scrubbing the toilet a few times a week with the toilet bowl brush helps prevent hard-water stains from developing.