Answer:
Assuming you have tried all the normal toilet cleaning products, the last resort is to use muriatic acid. It is normally used for adjusting the pH in swimming pools and can be found in the pool section of your hardware store. However, this is dangerous. You must use long rubber gloves and LOTS of ventilation. I have used this myself but only with the toilet outside while I was remodeling the bathroom. It works like you wouldn't believe though. Ten year old toilet looked like brand new.
White vinegar has been proven to work time and again to remove hard water stains. See the step by step method below:
Natural Formula to Use:
Pure white vinegar
Method to Use:
1.Fill a clean spray bottle with pure vinegar
2.Spray the vinegar freely over the toilet and wait for a few minutes
3.If stains are stubborn spray more vinegar and wait a while longer
4.Simply wipe away with a clean cloth to reveal the shine
Tips:
For toilet bowls add 1 liter of vinegar to the bowl itself and scrub with a toilet brush to dissolve away hard water stains that has built up over time. Also, by adding 1 liter of pure vinegar to the cistern (tank at back of toilet) this not only removes any stains from there but also removes any from underneath the rim on the next flush!
Sometimes stains are extra stubborn - for this make a paste of white vinegar and baking soda and scrub gently with a soft brush or sponge. Never use an abrasive cleaner like scouring pads, powders or steel wool as this will scratch the surface.