Frequently, iron and manganese which are found in almost all water distributions may cause staining in toilets or sinks. Often times the bleach products we use to clean our toilet bowls will cause the small amount of iron and manganese that is dissolved in the water to "fall out" of solution or become deposited on the surface of the bowl. As more water flushes by, the deposit builds until it becomes a stain.
The best method of minimizing stains is by scrubbing them away, however, be careful to not scratch the surface. Scratches make for the perfect environment for particles to embed and further exasperate the problem. For heavy-duty scaling, try a Lime-Away type product or a non-bleach cleanser. Pink staining is a form of biofilm which can be a nuisance as well. These stains generally show up when the water lies in the presence of soap. The tiny bacteria that causes the stain, feeds on soap scum. The best preventative action to take, if you experience this type of staining, is to rinse and wipe down the walls of the showers and / or sinks.