I recently surveyed a 3-year old pool about 300 feet from the ocean...nice property...but the owners' beautiful turquoise plaster has developed a splotchy dark stain (looks charcoal at first observance). After receiving the first phone call in which she described dark spots, I expected to find dark algae spots (customer said she almost never shocks and rarely tests!). However, I found a pool with clear water and in surprisingly good balance (7.5 pH, 4.5 ppm FC, 0 ppm CC, 60 ppm Alk, 200 ppm CaHard, 40 CYA, Temp. 77). However, they have been using 3" tri-chlor tabs in the skimmer basket, and I'm beginning to think that perhaps the years of acidic & chlorinated water hitting the pump when the timer comes on may have slowly added metal(s) to the water until they finally noticed this stain (a couple of months ago).
I have no experience with this particular pool, so I'm trying my best to be a Sherlock Holmes and figure out what is causing this problem.
My first idea about the culprit was colloidal silver algaecide (due to the blackness of the stain), but the customer said they have never used any. Their water sources are: rain (60 inches per year at this location), roof catchment (Architect 50 composite shingles running into a concrete tank painted with epoxy), and truck-delivered County water (seldom due to cost). They have a rarely used waterfall that runs into the pool over natural stone, tile, and perhaps a little bit of "mock rock" dark-stained concrete (maybe iron used in that stain?).
I have no source to purchase Citric Acid until Monday...so I thought I would upload photos to see if any of you have any ideas...note the bottom drain...no precipitating due to high speed water flow around edges? And around the drain, I note that I actually see tinges of yellow and red at the edges of the black. But the other photos only show black, no other colors. Weird...
As usual, thanks to you all in advance...it is such a blessing to have the privilege of drawing upon your collective experience!!