Magnesium, staining and calcium

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S

simonline

Member
Apr 12, 2021
13
Canberra, Australia
Pool Size
50000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Astral Viron V25
Been managing my pool succesfully since I inherited it 6 months ago.
Fibreglass, 50,000L, SWG salt water with sand filter.

Last weekend I did my monthly tests and got these results:
Free chlorine: 1.5
Combined chlorine: 0
pH: 7.5
Total alkalinity: 60
Calcium hardness: 225
CYA: 100
Salt: 4700
Temp: 15 degrees

I went down to the hardware store and got 6 litres (200 fl oz) of 4% bleach, 7kg (15.5lb) of calcium chloride, 100g (0.2lb) of soda ash and 12kg (26.5lb) of magnesium salt; as per the Pool Math app recommendations for the above levels.
I essentially threw it all in at the same time, dissolving in a bucket where necessary. The pool went a bit green which is normal for when you use magnesium for the first time (I wanted to try it out). Next day it was back to clear.

Fast forward 3 days and I have brown staining at the bottom of my pool, see photos below. The dark specs in the photos is organic matter and the darker blue on the edge step is the old colour of the pool as the light blue repaint was not a great job.

I found this site that says that Mg can oxidise when it comes in contact with chlorine and pH increases, both of which I did just before I added the Mg salt. It suggests that iron is the cause of brown but I don't know where I would have got excess iron from. Fiberglass Swimming Pool Staining Problem and Solutions
We've also had a bit of rain lately.

Assuming I do have heavy metal staining, how do I fix it please? It won't scrub off. Thanks!

I just did some tests today...
Free chlorine: 7
pH: 7.8
CYA: 75
I tried doing calcium hardness test but it kept failing and turning purple which the kit says it will do if there are too many metal ions in the water.


1634687351028.png

1634687404112.png
 
simonline said:
7kg (15.5lb) of calcium chloride, 100g (0.2lb) of soda ash and 12kg (26.5lb) of magnesium salt; as per the Pool Math app recommendations for the above levels.
Click to expand...
Where does Pool Math recommend magnesium?
simonline said:
I essentially threw it all in at the same time
Click to expand...
Really bad idea.

Never throw anything in at the same time, especially calcium chloride and sodium carbonate.
simonline said:
The pool went a bit green which is normal for when you use magnesium for the first time (I wanted to try it out).
Click to expand...
Where did you get that information?

The green was likely to be iron.
simonline said:
Fast forward 3 days and I have brown staining at the bottom of my pool
Click to expand...
Try vitamin C on a spot.
simonline said:
I found this site that says that Mg can oxidise when it comes in contact with chlorine
Click to expand...
What site was that?

Magnesium in magnesium chloride is fully oxidized and cannot be oxidized further.
 
This is the magnesium atom.*

There are 2 electrons in the outer valence electron shell for the element.

For the ion that is in magnesium chloride, the two electrons are gone and the atom cannot be further oxidized.

Oxidized means to lose electrons.

1634705180329.png
*Actual picture enlarged to show detail.
 
JamesW said:
Where does Pool Math recommend magnesium?
Click to expand...
You're right, it recommends salt. I was hoping that it didn't really matter what type of salt.

JamesW said:
Really bad idea.

Never throw anything in at the same time, especially calcium chloride and sodium carbonate.
Click to expand...
Not exactly at the same time, they were all each about 15 mins apart as needed to prep the next thing and waited for the previous to dissolve.

JamesW said:
Where did you get that information?

The green was likely to be iron.
Click to expand...
The reviews on the site that I got the Magnesium from: Hy-Clor 12.5kg Mag Salt Pool Chemical

JamesW said:
Try vitamin C on a spot.
Click to expand...
This worked!

JamesW said:
What site was that?

Magnesium in magnesium chloride is fully oxidized and cannot be oxidized further.
Click to expand...
The link is in my original post, it's from the site thesummerpools.com

Thanks for the reply James, given that the vit C trick worked, it looks like I have iron but not sure from where. I'll start researching how to reduce iron in my pool water and figure out where it came from.

Just remembered that I recently unscrewed the pool light to replace the globe and there was a lot of brown rust around the metal frame and screws. I rubbed it all off into the pool (the frame has to remain submerged) but wouldn't have thought that would have been enough metal to impact the pool composition.
 
simonline said:
Been managing my pool succesfully since I inherited it 6 months ago.
Fibreglass, 50,000L, SWG salt water with sand filter.

Last weekend I did my monthly tests and got these results:
Free chlorine: 1.5
Combined chlorine: 0
pH: 7.5
Total alkalinity: 60
Calcium hardness: 225
CYA: 100
Salt: 4700
Temp: 15 degrees
Click to expand...
lucky it was winter in australia
i feel you are heading for a wreck
pool store testing, fc 1.5 with cya 100
only testing once per month
suggest you get a quality test kit
taylor k2006c or clear choice labs
apart from the bleach you didn't need any of the other chems. if those numbers are believable
in australia liquid chlorine from pool store or big brand hardware stores is cheaper than bleach
 
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aussieta said:
lucky it was winter in australia
i feel you are heading for a wreck
pool store testing, fc 1.5 with cya 100
only testing once per month
suggest you get a quality test kit
taylor k2006c or clear choice labs
apart from the bleach you didn't need any of the other chems. if those numbers are believable
in australia liquid chlorine from pool store or big brand hardware stores is cheaper than bleach
Click to expand...
I test myself using the Total Pool Test Kit from CCL. Been using that kit since I got the pool so am getting reasonably good at using it, I think. (I did 1st year chem at uni if that counts for anything). I don't have time to test more often as I am renovating the house. I'll try to go to fortnightly testing when we actually start using the pool.
I log the results from the CCL kit into the Pool Math app and then follow the recommendations the app tells me.
Glitz Bleach is $1.70 for 2L and Hy-Clor Liquid Chlorine is $15 for 5L at Bunnings; way cheaper to get bleach.

James, you're right it doesn't however my SWG model recommends 6000 salt level which is what I told Pool Math to aim for in the recommended range as you can set that yourself.

I never said that Bunnings recommended Mag Salt, just that the reviews said that it can turn your pool green so I expected that to happen. Bunnings is like Lowes or Home Depot of Australia, I'm sure they are fairly impartial as to what salt you buy, as long as you buy it from them!
 
JamesW said:
Probably has a lot of iron in the product.

email Hyclor and ask if the product contains any iron or yellow prussiate of soda.

[email protected]
Click to expand...
In Aus you have to have an MSDS with all chemicals. For this product the composition is...
Magnesium chloride, hexadyrate: >95%
Ingredients determined to be Non hazardous: Balance
Maybe the <5% "balance" has some iron in it?

Anyway, thanks for weighing in, I'll treat it as excess iron in the water and try to test more often.
 
I am confused about how the cya dropped so much in 3 days. Are u testing cya yourself with a turbidity test? Did u exchange 25% of your pool water?

Testing fc,cc, & ph should be done every couple days or so or you’re asking for trouble. Fortnightly is quite a stretch.
TA once a week or so if it stays pretty much the same & you’re not actively trying to adjust it or using acidic products that lower your ph below 7.2.
Cya & ch don’t change much unless there’s alot of water exchanging so every couple/few weeks is ok.
 

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simonline said:
I test myself using the Total Pool Test Kit from CCL. Been using that kit since I got the pool so am getting reasonably good at using it, I think. (I did 1st year chem at uni if that counts for anything). I don't have time to test more often as I am renovating the house. I'll try to go to fortnightly testing when we actually start using the pool.
I log the results from the CCL kit into the Pool Math app and then follow the recommendations the app tells me.
Glitz Bleach is $1.70 for 2L and Hy-Clor Liquid Chlorine is $15 for 5L at Bunnings; way cheaper to get bleach.

James, you're right it doesn't however my SWG model recommends 6000 salt level which is what I told Pool Math to aim for in the recommended range as you can set that yourself.

I never said that Bunnings recommended Mag Salt, just that the reviews said that it can turn your pool green so I expected that to happen. Bunnings is like Lowes or Home Depot of Australia, I'm sure they are fairly impartial as to what salt you buy, as long as you buy it from them!
Click to expand...

False economy Simon. Liquid chlorine is 12.5%, 3X stronger. You would need 45L of the bleach to equal the same free chlorine which would be close to $40 worth. You don’t need to tell the misus why you decided to switch to pool chlorine.;)

Just my 2 cents but I think magnesium should be confined to the bath tub. It offers nothing for a swimming pool. Costs more and interferes with the CH test. The top quality standard pool salt is the one you want, the cheaper on has more impurities and the better quality one isn’t that much more and dissolves better.
 
Thanks for all your contributions. I ended up hitting the stain with a sock full of vitamin C and it took care of it all.
Thanks also for the tips on testing and the difference between liquid chlorine and bleach. I think I need to upgrade my testing kit equipment if I am going to be testing that often!
 
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