Many water sources contain high levels of iron, which can lead to unattractive yellow or brown stains on your pool's surfaces. With frequent pool cleaning, these can be avoided. If they do become set, however, you can use a simple chlorine solution to get your pool looking like new again.
Cleaning your Pool
Wet the bristles of your brush and dust them with a thin coating of powdered chlorine. This is the best chemical to use because it is powerful and will not contaminate your pool.
Scrub this over the stain, reapplying the chlorine as necessary, until the stain is removed.
If the stain still will not come up, you may need to drain the pool and scrub it with a strong solution of liquid chlorine.