Help with fibreglass stain

Welcome to TFP.

Your mates, @mgtfp @AUSpool , understand the magnesium chemistry you use there.

I would say it looks like metals stain.

Welcome!

I tend to think metal, too.

You have ruled out iron by trying vitamin C. You can try some dry acid in a sock to test for copper. Have you used copper containing algaecides? Or other products like "multifunction" chlorine tabs? They usually contain copper. Or impurities, or even deliberately added "minerals" in the magnesium salt.

I'd recommend to order a test kit from Clear Choice Labs. This has a reliable FAS/DPD chlorine titration test that is vital for the TFP method. Take a full set of tests with that. To see the likelihood for algae we need to know your FC and CYA levels.

Have you only added magnesium salt or also regular sodium chloride? I assume you have an SWG?

We are not huge fans of the magnesium hype in Australia. These salts often contain Epsom Salt, which is magnesium sulphate. Sulphates create over time a corrosive environment which is detrimental to SWGs. Also bad for plaster surfaces. High levels of magnesium can also interfere with the calcium test.

Geday Jass and welcome to TFP,

A single shock treatment is never enough, particularly if you run on the low side of FC with CyA. TFP,s
SLAM Process is basically a shock treatment using pool chlorine that continues until all algae is gone. Hopefully your stains are persistent algae, as a minimum I would recomended getting the FAS/DPD FC kit and a speed stir from clear choice labs and run a slam.

It sounds like you have ruled out iron which is a common impurity of magnesium mineral salts. When adding the mineral salts did it ever sit in the spots where you have the stains?

I am reluctant to offer any advice in general about stain removal, particularly with a fiberglass pool where any number of the potions can void your warrantee. A question for your pool shell manufacture maybe? But be cautious there because a single shock treatment can be enough to void your warranty.

It's difficult to give advice based on this test. CYA appears to be really low. If it was truely zero, then FC would be right, if it was 10, then this FC would be nothing. But I don't really want to say to add more Stabiliser based on a test result I don't have confidence in.

My suggestion would be to get that kit, and run a full set of tests. And then do an Overnight Chlorine Loss Test, that'll show us if you are dealing with algae.

Is your pool pure fiberglass or do you have a waterline tile with cement grouting?

From the Poolrite site the range for their chlorinators is given as 3500-7000ppm.

The purity of the expensive mag salts is less then regular and they have a higher chance of having iron as an impurity. Regular AU salts are 99.4%, most mag salts are 98% while the Bunnings one is 95%. When it comes to salt a “bag” is not exactly an SI unit and the confuse things our regular salt a sold in 20kg bags, the mag salts are normally 10kg while the bunnings one is 12.5kg. I like to use kg’s.

Reading the history the stains were inherited with the pool and you’ve tried just about every concoction they make. I would send an email off to the pool stain removers with that history and ask their advice.

It looks like you’ve been chasing a low pH and probably a low FC. If you ask a pool store or fibreglass manufacturer why they recommend that you get a blank stare. No one has ever given me a satisfactory reason. A low FC will minimise the reactive form of FC as hypochlorous acid but a low pH maximises it within that FC level. TFP is about self testing for reliable results and then selecting target levels that are based on real scientific, peer reviewed and published literature that helps to make pool care easy. You can run with a pH of 7.8 and a TA of 70ppm to minimise acid use. CyA and FC should always be treated as combination pair as per the
FC/CYA Levels and never as independent levels. The methodology here has been tried and tested over and over, TFP has over 300,000 members which makes even the largest Facebook group look like a little tea party.

In the end, this is an optical problem. We have bought a house with a 40 year old plaster pool. Certainly beyond its prime. But hey, I've got a hole that holds water. And over the years I have come around to actually understanding what's going on in my pool, rather than just dumping to n what the pool store tells me.

Eventually my pool will see a renovation, and then I'll hopefully not stuff it up immediately after just having fixed it.

It might be a good idea to take a step back, go through all the resources here and apply that in your pool maintenance routine. Once that all makes sense and you gained some confidence in your own testing, you'll probably have picked up enough knowledge to take the next step in attacking those stains. And keep them away.

The worst thing is to create an even bigger problem by trying to fix a smaller problem.