When you see rust, you know there is metal involved and it can occur in a plaster, tiled or fibreglass pool. The stain can start out as a little spot on the wall or bottom of your pool. Know what could possibly be causing it and what can you do make it go away, or keep it from happening in the first place.
Oxidised Iron in the Water
The water you use to fill your pool can have metal in it. If you are seeing many tiny rusty spots, then that is probably the case. Adding a "metal out" additive to filter the water is your best defence for this issue. You may find you need to sweep and vacuum more often as well. There also are ways to acid wash the spots directly, or you can scrub them with a brush as needed. Some people also use magnets in their pools to collect tiny bits of metal.
Soluble and Colloidal Iron
"Clear water" or soluble iron is drawn in from a well and rusts when it comes into contact with the air. This forms particles in the water that are reddish brown. This problem can be solved by feeding chemicals into the water to oxidise the iron, which can then be filtered out with a mechanical filter. You also can add chemicals that basically coat the iron so it will not receive oxygen. Colloidal iron also is rust that is in the water, but it's usually bound together with other substances. To deal with this, either add chlorine to oxidise the iron from the other substances, or add polymers that will ultimately cause the iron to form larger clumps that can be filtered away.
Bacterial and Organic Bound
An iron bacterial problem is due to living organisms. They like to feed on iron that is found in water and build slime along the path of the water. It will appear rust coloured, but also can be green and must be killed using high amounts of chlorination throughout the plumbing to kill it. An organic bound pool takes place when tannins and other organics combine with iron. This type often is mistaken for colloidal rust, but cannot be removed by oxidising or ion exchange filters. Test for the presence of tannins. If present, tannins can be removed by adding chlorine and by using a carbon filter, but the carbon bed must be replaced when saturated.
Rebar and Concrete Rust
Under the plaster of your pool are intersecting metal shafts called rebar. Rebar will rust through if placed within an inch of the surface. Untreated, rusting rebar can create a bigger issue, because the corrosion always is worse underneath. The concrete used in your pool also could be the culprit; sometimes there are trace amounts of iron within the mix. Untreated, it could spread throughout the pool and cost much more to repair. While you can call a professional, some choose to cut out the rusted section and patch the pool with grey masonry cement. It can then be recovered with plaster or tile.
Metal Items in the Pool
Simply dropping a metal item in the pool can cause rust stains. It could be a nail, hair clip or a pop can. Any metal item that can rust also can stain your pool. Make sure that such items are removed immediately upon being discovered. The more acid in the pool, the faster the problem becomes but if you do end up with a stain, clean it up using a steel brush or a rust removing liquid that can be purchased at any pool supply store.