Thousands of strains of mold grow on Earth -- perhaps as many as 300,000 -- and they have one thing in common; They need moisture to survive. There's plenty of moisture inside a toilet bowl, especially if you keep the lid closed, and mold commonly grows there -- but not toxic black mold, because it feeds on cellulose. All that discolors the porcelain is not mold -- minerals in the water are also responsible, and they are more difficult to remove.
What's That Strange Color?
Mold that grows in your toilet can be green, orange, brownish or pink; it can even be black, but the black mold in your toilet isn't Stachybotrys chartarum, commonly known as toxic black mold. You'll find that strain on wet walls or in damp framing -- where cellulose is plentiful -- but not in your toilet. Besides mold, minerals also discolor your toilet; manganese leaves black rings, iron leaves rust-colored ones and calcium leaves yellowish ones.
Getting Rid of Mold
Mold removal isn't as difficult as you might think. Bleach kills mold and is effective on non-porous surfaces like porcelain, but you really don't need anything more than detergent and water, according to guidelines published by the U.S. Environmental Protection Agency. The key is to physically remove the mold by scrubbing, and you can do this with a conventional toilet cleaning product and a toilet brush. Be sure to scrub thoroughly, especially under the rim and behind the toilet seat cover.
Deep-Cleaning Your Toilet
By itself, neither bleach nor detergent will make much headway against mineral deposits. For these to dissolve, you need an acid, and the strength of the acid you need depends on the severity of the stains. To restore your toilet bowl to its original color, follow a simple cleaning strategy:
Mix a solution consisting of 1 quart vinegar, 1 quart water and an ounce of laundry detergent. You can substitute lemon juice for vinegar, if you prefer.
Scrub the inside of the toilet bowl, using this solution. After scrubbing, cover stubborn stains with the solution and leave it for 15 minutes to an hour to give it time to dissolve the minerals, then scrub again.
Make a paste using vinegar and borax and use it to cover stains that you can't remove. Leave it overnight, spraying occasionally with full-strength vinegar or lemon juice to prevent it from drying out. Scrub again.
Use a commercial limescale remover to take care of really tough stains. Commercial products usually contain stronger cleaning agents, such as phosphoric or hydrochloric acid, and work more quickly. Wipe the product on, leave it for the recommended time, then wipe it off. Wear rubber gloves and goggles.
Rinse the toilet bowl with water, then sprinkle some baking soda on the porcelain and scrub it with the toilet brush. This effectively removes any leftover residue and deodorizes the bowl.