Black stains on vinyl or fiber-glass lined pools.

Black stains on a vinyl lined or fiberglass lined pool indicates either algae or copper staining.

If the pool has a chlorine reading of anything greater than 0.2 ppm and the pH is close to 7.0 then it is unlikely to be algae.

Copper staining comes from previous use of copper based algaecides. It may also be a result from brass or copper fittings in the pool and the owner has added too much acid acid over a period of time and dissolved copper into the pool water. (1 to 2 grams or the equivalent of the smallest coin) .

At a pH of 7.4 and in the presence of chlorine, copper turns to basic copper carbonate (sometimes the pool assumes a funny green colour at first). Further addition of copper converts the basic copper carbonate to copper oxide and the pool walls stain black or smokey-grey.

About 1/2 kg of citric acid sprinkled down the areas of staining will remove and dissolve the stains immediately. If this does not remove the stain then it is not copper.

If the pool has trees or greenery over it then it may be either manganese or iron from the shrubbery falling into the water. Manganese and iron stains are best removed with Oxalic Acid. (In Australia, the local hardware sells the material as "brick and tile cleaner")