Help with identifying and treating brown stain

[Beakman]

We rented out our house for a year and left the pool in the care of a pool service for our renters. Upon our return, I was dismayed to find lots of brown staining all over our pool. The chemistry of the pool was all out-of-whack when we returned, so I am left to believe that the pool was not well cared for in our absence, but have no real idea how long the stains have been there or what crazy things they did with the pool chemistry.

Pool is 20,000 gallon pool, PebbleSheen, with in-floor cleaning and a SWG.

Current pool chemistry:

FC: 4
CC: 0
pH: 7.4
Stabilizer: 100
Alklinity: 120
Salt: 3500 ppm


The stains are brownish colored, like dirt, and are all over the pool seemingly at random. Near many of the in-floor cleaners there is a clear fan-shaped pattern where the immediate path where the water squirts is fine, but in-between the "stops" is the brownish stain. There is also similar brown staining in places like corners and on top of steps where dirt might naturally collect. It looks like a bunch of dirt blew in and nobody cleaned it, expect water is clear and the dirt doesn't brush away.

I have tried shocking the pool (up to 20 FC) with no real impact. I have tried applying vitamin C tablets in several places with no noticeable change. I lowered the pH down to 7 and brushed like crazy with a stainless brush for a day, again no change.

There was noticeable scale buildup at the tile line and in the SWG. My best guess at this point is that maybe it is dirt that got covered up with scale, but I'm not really sure how to test that theory, nor how I might treat it.

Short of an acid wash, any suggestions on what I can try?

[PoolDoc]

Sorry about the delay -- I can't seem to get caught up.

If you've tried Vitamin C, and have tried putting chlorine powder on the spot . . . then dirt is a good guess. If you want to check out every possibility, you can try the
JACKS MAGIC STAIN IDENTIFICATION KIT

If their kit can lift a spot, there's a fair chance they may have a product that can do the same, on a larger scale. They tend to be a bit overpriced, but it would be cheaper than acid washing the pool.