Ways to remove calcium include using vinegar or lemon juice, a plumber's cloth, commercial toilet-bowl cleaner and muriatic acid. Another method is to use a pumice stone.
Vinegar and lemon juice are two natural options for dealing with calcium deposits. If the calcium buildup is significant, the liquids may have to be used more than once. An individual who chooses to use a plumber's cloth only has to dip it in water and scrub away at the deposit.
While using a commercial toilet-bowl cleaner for calcium deposits, it might be better to completely drain the toilet beforehand to keep from diluting the cleaner with bowl water. Tank drop-ins offer another viable option for preventing stains in a toilet bowl, but they work over time rather than instantly.
While muriatic acid is highly effective against calcium deposits, it's also very dangerous and harsh on porcelain. It's best to use the chemical in a well-ventilated area along with protective gear for the hands and eyes. Five gallons of water should be added to the bowl along with 12 ounces of muriatic acid, which should be poured with caution. A long-handled toilet brush can be used to spread the acid around. This might need to be done several times before the calcium is eliminated.