Doh. I'm in New Zealand, I can't even get ascorbic acid here - I doubt they have heard of revese osmosis. Thanks anyway.

I agree it is probably iron. I have the same problem. You will need a life time supply of metal sequestrant.
My tap water tests negative for iron so I suspect my iron comes from the large quantities of leaves which fall into my pool. Tea anyone?

If you can't get ascorbic acid, try the others mentioned. Can you get Bar Keepers Friend (a powdered organic acid cleaner)? You can use that.
A drug store may be able to order the ascorbi acid as a special order.

Once the iron is precipitated on your pool wall, the water will most likely test negative because all the iron is on the pool wall!

Okay, so I'm getting a better idea of the problem and I'm working on a solution. Can I get some clarity on the order of things, I'm not 100% sure about how it works:

* Use ascorbic acid or similar to drop the iron into solution.
* Then I need something to 'capture' the iron to stop it replating at some point - is that the job of a sequestrant? I can get Bioguard's 'Metal Magnet' here, is that considered a sequestrant (what is a sequestrant chemically? (so I know what to look for on the label)
* Once sequestred either dump and refill or flock and vac (I like the 2nd idea)
Then maintain a level of sequestrant as part of my ongoing maintenance.

Does this make sense assuming I keep an eye on my levels?

Woohoo. I have found and ordered some ascorbic acid here in New Zealand, should arrive in a day or two.

I was going through some other posts about iron stains and read about one person who had a washer cause rust marks in his pool. It got me to thinking about some metal hair clips that left rust marks on my pool and a couple of small nails my young son dropped in. Is it possible that these items could cause such an impact on a pool?

I think I have found a sequestrant here too. It is by an Australian company called BioGuard, the product is called Metal Magnet and is described as :

BioGuard® Pool Magnet™ is a chelating agent that
binds iron, copper and manganese in the water to
prevent them from being oxidised and staining the
pool surface or discolouring the water.
Product Features:
• Liquid chelating agent.
• Helps prevent staining from iron, copper and
manganese.
• Helps remove fresh stains.
• Eliminates water discolouration from oxidised
metals.
• When used according to label instructions, product
remains effective for up to 4 weeks.

They refer to it as a Chelating agent. Is that the same at a sequestrant? I can't tell if it uses phosphonic acid though. Does anyone know of this product?

Thanks again!

The msds says it contains '1-Hydroxyethylidene-1,1-diphosphonic acid'. Looks like phosphonic acid to me.

Chelating and sequestering are interchangable in this case.

Yup, that's 1-Hydroxyethylidene-1,1-diphosphonic acid aka HEDP.

A follow up.

So a few weeks later I have some good news and bad.

I eventually got 1kg of ascorbic acid and it did the job, the stains vanished within an hour. I added a sequestrant and a special polymer? powder that is supposed to bind the sequestered iron and flush to waste. Anyhow, all owrked well.

During all of this my chlorine levels went very low - fine - but I had some algae developing. Once the acid had done it's job my pool was looking decidedly murky but hey, I was happy as it was now a cloudy green rather than a brown murk.

I slowly bought my levels up to recommended but I still couldn't clear the fog. I relented and switched from a Polymeric Algaecide to a copper based one, just to see if things changed. I also hit the chlorine harder. Guess what.......the iron stain has returned.

Bugger. I'm going to start again with ascorbic acid and then narrow down if it was the chlorine that caused the stain to come back or if it was the copper based algaecide.

I'll keep going but any thoughts would be welcome. I'm busy going through all the posts on controlling algae.

Since you already have a stain problem the worst thing you can do is add more metals. Those 'binding' compounds don't really work very well. And now you've added copper to the mix. Not to be harsh, but there's a reason we recommend the things we do. Once you stop doing things we don't recommend and start doing the things we do recommend you can have a clear and trouble free pool. It's like chasing your tail to use copper in a pool that already has staining issues. Add copper, more sequestrant, add copper, more sequestrant, add copper, more sequestrant. you get the picture.

Raise your dose of sequestrant and then shock the pool. if the stains return add more sequestrant. it may take a good bit until you're done shocking.

P.S. STOP adding metals!

With the CYA level reported at 160 ppm, partial drain/refills or an overall drain/refill would be needed anyway so I would just use the ascorbic acid which seemed to work to remove the stains and then remove and replace most of the water via multiple partial drain/refills if an in-ground pool or a nearly complete drain/refill if above-ground. The ascorbic acid will reduce the iron to soluble form so when you remove the water you will be removing the iron as well. The pool is 8000 gallons so while its still a lot of water, it's not as much as in larger pools. See koos' thread Have you ever started from scratch.

If the fill water is high in iron content, then an HEDP metal sequestrant should be used and the chlorine level raised SLOWLY to prevent new metal staining.

I suspect that perhaps Trichlor tabs/pucks have been used which explains the high CYA level and maybe they were used in the skimmer and the low pH has corroded some metal parts in the equipment. Either that or a Trichlor floating dispenser may have parked itself near a railing and corroded it. If the fill water is from a municipal water district, then get a water report which would show you the iron content. If you are filling from well water, then it could very well be high in iron content.