Black Spots on Fibreglass Swimming Pools.
Most fibreglass pools will, at some stage, develop black spots on the surface of the pool.
This type of black-spot is not a bacteria but a mineral that bleeds from the inner skin of the fibreglass.
It’s appearance can be delayed and controlled if the water Ph level is kept around 7.2
Some advise on Black-spot treatments from:
http://www.aquahouse.com.au/Pool%20M…20treating.htm