Stains in swimming pools are almost always caused by metals in the source water. The metals come out of solution in one or more place on the shell of the pool and can seem to be impossible to remove by conventional cleaning. The stains are usually brown in colour but can be yellowy or black. Fibreglass pools seem to be more prone to staining than any other type of pool.
The removal of stains is a 2 step process. First you have to get the stain off the wall by getting the metals out of there and in to the water. Then you have to get the metals out of the water.
Start by adding Multi Stain Remover to the water. This is quite a strong acid and will take the metals out of the walls and in to the water.
The second stage is to remove the metals from the water using No More Metal. The metals are held in suspension so they can be filtered out. No More Metal should be added on a regular basis thereafter to keep the level of metals down in the pool water and so prevent the stains from re-forming.
Stain removal is not a simple process so read the instructions carefully. One thing to note is that the first product is very acidic and will alter the pH balance of the pool. When re-balancing the water you must do it gradually over a few days or you will un-do the work that the stain remover did and the stains will re-form.
For more information on the products mentioned follow the links below