I drained a small (1000 gallon) kiddie pool adjacent to the main pool to remove some iron stains and took the opportunity to examine the light colored stains in smaller pool. While there were a few small scale marks (easily removable by diluted muriatic acid) the more numerous light colored stains appeared, on close inspection, to be slight indentations where the lighter colored underlying concrete was showing through the darker plaster. I presume this would be the same pattern for the larger pool if I chose to empty it.

My assumption is that the acid wash performed 3 years ago actually removed not only the scale marks but also the underlying plaster. Does this seem a plausible explanation? It is hard to know whether this was a botched job or an over vigorous wash on an ageing pool surface. Unfortunately it seems likely I will need to get the pool re-plastered if I am to get any improvements on my present situation. Any thoughts?

On a practical note I have discovered that if I turn all the return nozzles to face upwards the rippled water surface hides the discolorations at the bottom of the pool.