Black Stains on Vinyl or Fibreglass 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 the use of copper algaecides in the past or if there is brass or copper fittings in the pool and the owner has added too much acid acid over a period of time and dissolved some 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 (the pool assumes a funny green colour first sometimes) 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")