No matter what type of swimming pool you have, whether it is a gunite, fiberglass, or vinyl liner swimming pool, you can get stains that may ruin the look of your pool. The stains can be caused by organic materials such as plants, leaves, and even small branches falling into your pool. Stains caused by organic items have a tendency to be black or a tann-ish brown, almost rust-colored stain. Metals can also cause staining in your swimming pool, especially if you are using well water. However, metals such as magnesium, iron, and copper can be found in all types of water.