Metal stains, Algae, Mottling, or other?

I'm a fairly new pool owner that I inherited from the previous homeowner and I've used this forum almost exclusively to learn as much as I can.

There's a weird dark purplish discoloration on the pool walls and floor that I don't believe were there when we moved in 3 months ago. I rubbed a chlorine puck on it and there was no change. I used a putty knife to scrape some of it off and it was sandy and purple-ish in color. Prior to learning about CYA we used chlorine pucks and powdered shock only to have our CYA levels and hardness off the charts (CYA 200+ according to a test strip). We have a chemical test kit also but it doesn't test for CYA. We have a pool company coming out to look at it this week but I figured I'd see if any of the knowledgeable people here can help identify the discoloration. My guess is it's some sort of mineral deposit? Here are some pictures if that helps any.



TFP Guide
In The Industry
Apr 5, 2015
tucson, arizona
The purplish color is a stain usually caused by the combination of copper and high levels of cya. You also have a good amount of soft spotting, which is the white spots and lines in the picture


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
Aug 10, 2012
We've got a little work to do.

First, you're going to need a good test kit. I prefer the TF-100 from It is the best value, here's why, Pool School - Test Kits Compared

Once you get the test kit test your CYA and do some partial drains to lower CYA down to 50 or so. And then the pool to kill the algae. Once the is completed we can work on the metal stains.

Do a little reading in Pool School, here are two good articles to start with

Pool School - Getting Started

More about stains here, Pool School - Stains in Your Pool


New member
Jan 18, 2017
New Orleans, Louisiana
Thanks guys!

The pool cleaning company came out yesterday and said it's definitely mineral deposits and not to worry too much. My two options were to drain and acid wash (which he didn't feel was necessary), or wait until the weather warms up some and they would be able to get in and scrub. My CYA was almost 200 so I drained about 5,000 gallons out and refilled. I'm going to take a water sample down to the pool store to see what impact that made. I will definitely be investing in a CYA test kit!