I recently purchased a home with a 7-year old in ground pool with a salt water system. For the most part the pool and equipment are in very good shape, except for a high calcium hardness reading and two different kinds of plaster stains.
First my current test results:
FC = 5
CC = 0
PH = 7.2
TA = 70
CH = 650
CYA = 60
Salt (per test strip) 3080
When I first moved into the house (about 1 month ago) the PH was over 8.0, TA was 100 and the FC was at 10. For all I know, this may have been the situation for quite some time because the prior homeowner had the water tested at the local pool supply store about once every 2 weeks or so.
Anyway, thanks to this forum and the TFT test kit I was able to monitor the water on a daily basis until I got it (somewhat) under control. I definitely intend on fixing the CH as soon as I get the chance to drain some of the water and then refill it. The CH of my source water is 50ppm so partially draining and refilling should help dramatically. I currently have calcium scaling on the tiles and at other places at the waterline.
Now for the plaster issues:
I've got dark and blotchy type stains near two of my returns as seen in this pic:
Some parts of the stain appear to be brown (like dirt, especially around the perimeter of the return), but for the most part they're just dark gray (at least to my eye). I tried vitamin c in a sock and it seemed to lighten it, but very little. I also tried a pumice stone, but that didn't do anything. I did not try a chlorine tablet because the FC was super high for quite some time and that didn't seem to lighten anything.
I have an attached hot tub with a waterfall, but the plaster in the tub does not have any of this staining.
The second issue with the plaster are these white marks (see stairs especially) which are pretty much everywhere throughout the pool on the walls and floor (but nowhere near as prolific in the hot tub):
Here is a close up of a landing which is under about 6 inches of water:
Neither the stain or the white marks are rough to the touch. During the day in direct sunlight, you really have to look good to even spot them. It's only in the morning or at dusk when you can see them very well.
Any who, I'm thinking that the aggressive water had something to do with both of these issues? And then the next question of course is, what can be done about them?