Is it scale or something else?


Bronze Supporter
Jan 30, 2017
Houston TX
Greetings, all.

We just had the pool emptied and acid washed to deal with a yellowish gray scale that started within weeks of first filling the pool. and got worse through the first year despite balanced water values. The acid wash seemed to help a lot, but within 7-10 days, I'm already noticing it again around the steps and along the bottom of the wall in the shallow end (same place it started the first time). I thought maybe it had been an issue with the pH management the first time around (start up handled by PB), but we've been managing it very closely (consistently at 7.8 or lower within first two days) and it's happening anyway.

I've read up a bit on the three types of scale. Based on appearance and response to lowering pH/brushing, it doesn't seem to be calcium carbonate. So I'm trying to figure out if it's calcium phosphate or calcium sulfate. Is there a test for the water itself to figure out if phosphate or sulfate values are out of wack? Any ideas for what can be done to get it under control? Or is it something else entirely?

General timeline:
-Summer 2018 - gunite done by reputable sub
-Winter 2018 - gunite surfaced with PebbleSheen by same reputable sub
-PB handled water balancing for the first several weeks, and we did multiple brushing/day, cleaning skimmers.
-Within the first two to three weeks I noticed a yellow-gray coloration near sloped areas where vertical and horizontal surfaces met (around steps, along wall in shallow end). At first I figured it was how light reflected or how pebbles were distributed. Over time it seemed to spread throughout surfaces in pool despite keeping water well-balanced.
-The sub came out and said that he could see it (scaling). He recommended dropping pH to 7.0-7.2, continuing to brush frequently, and using JM The Magenta. If it didn't work, they would do a partial or full acid wash. We followed the instructions, but it did not go away. If anything, it seemed to increase. At that point, we let the pH go back to range in Pool Math.
-After several postponements due to rain, the sub came back on Nov 19 to do a thermal treatment (said it looked like there may have been water trapped behind surface) and an acid wash in drained pool.
-The pool was filled immediately and after the green cast from the fill water passed, it looks noticeably bluer (Aqua Blue PS).
-However, within a week of the acid wash, I'm already seeing the yellow-gray cast in those same areas (near steps, wall/floor slope of shallow end).

Thanks for any insight you can provide!