I just noticed this today. I had a new pebble finish put in back in June/July and have kept the PH around 7.2 till Dec/Jan only recently did I move it up to a more normal level. It is currently 7.5, and my Cal is at 350ppm. I just noticed today some sort of white rings developing around the perimeter of the individual pebbles. Anyone know what this is? or what to do about it? In some places its really not that noticeable and in others its worse. It seems to be the most pronounced right below the skimmer.
IMG_2252 by maleora99, on Flickr
IMG_2251 by maleora99, on Flickr
IMG_2250 by maleora99, on Flickr