A "Shocking" Cautionary Tale & Advice Needed Please

Mandy

Silver Supporter
Jul 5, 2017
34
Dubbo Australia
Hi all. Just a brief background……..I knew the pool required a salt top-up as I couldn’t raise the SWG above a certain level, and we’ve recently had some good rain (yay for our drought and dust bowl town). I also knew my CYA would be low. Due to family and other commitments I’ve been busy with other things and neglected my “baby”. I’d been adding acid to keep the pH in check based off my experience in the past (read that as I estimated how much to use instead of testing first).

Yesterday I decided to show my pool some love so out came my test kit and these were the numbers:

FC 1.5
CC 0.5
pH 7.6
TA 80
CH 190
CYA 0
SALT 1000
Temp 21°C
CSI -0.26

So I did the following:

Added 300gm 73% Cal-Hypo to raise FC by 8 (aimed for this as I was planning on adding salt later in the day and would need to turn SWG off for 24 hours)

45 minutes later added 620gms CYA to raise to 20ppm – the first of three additions. Planning on 2 more at 500gms each to get to a total of 60 in coming days. Adding over time so as to not cause pH to drop too low. Will recheck after that and adjust to 70. This first addition would have lowered the pH to 7.2

Checked FC and it was reading 10ppm

8 hours later I rechecked FC it was at 7.0

Added 112gms of 73% Cal-Hypo to raise FC by 3 (to keep my buffer)

Shortly afterwards I wasn’t sure if the water took on a very slight greenish appearance, I thought maybe it was the reflection of the vine growing on the side fence plus it was close to sunset

30 minutes later I turned off the SWG and added 100kgs of salt to increase it by 3700ppm

During this time I could see the water had in fact started to look a greenish colour plus there appeared to be some very fine dusty stuff settling on the bottom at the deepest end of the pool. As I swept the salt to help dissolve it the water mixed with this stuff and became more green in appearance.

My FC demand for the day had been 3ppm and prior to adding the second lot of Cal-Hypo the water had looked clear, clean and sparkling. I’m thinking I "shocked" the pool (in more ways than one!) and the green tone was from iron that had come out of solution (not sure if that’s the correct term for this).

I filled the skimmer basket with poly fill and left it overnight.

This morning the poly fill was very yellow/brownish in colour and the water was a little less green looking. I checked FC it was 9.5 (so passed OCLT) and pH was 7.5

I have been running the vacuum all day to stir up the water to assist in filtering out the green and the residue that was sitting on the pool floor plus have been changing the poly fill and adding heaps of paper towels on top of it in the skimmer. These are definitely capturing a yellow/brown stain.

As of now the water is appearing a lot less green in tone and is very clear. I suspect it will be back to TFP beautiful by morning.

I’m 99% (if not 100%) certain this is not an algae attack.

So this is where I need some advice and clarification around what happened. I’m thinking the metals precipitated out due to raising my FC level too fast and also too high for my CYA level. Is this possible?

If I had added the CYA first, followed by the chlorine would this still have happened?

Could the addition of chlorine, CYA and salt all on the same day have caused this? If so, why?

Is it possible metals coming out of solution can fall to the bottom of the pool as very fine dusty stuff?

Why are the metals turning the water green as opposed to attaching to the fibreglass surface and causing the pool surface to turn brown (as I’ve experienced in the past)?

Could this build-up of iron have been introduced into the water via the numerous dust storms we have been experiencing? Our top-up water is pre-filtered using 2 Pentek filters.

I ceased using a sequestrant 2 years ago and since then there has been a very slow increase in the number of tiny brown stains in various spots around the pool but not enough to warrant a stain removal treatment.

If the iron is now visible in the water should the brown iron stains that were already on the fibreglass now be gone? They are not.

I'm holding off with my next dose of CYA until all the green has filtered out. A bit concerned that lowing the pH now might cause a problem. Could it?

I’m not looking to be advised to use a sequestrant to bind with the metals – I just want to understand why this happened and would be very grateful for any and all advice thank you.
 

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Bama Rambler

Mod Squad
TFP Expert
Bronze Supporter
LifeTime Supporter
Jun 22, 2009
23,873
SouthWest Alabama
Raising the FC that high with no CYA reduced the iron in the water and it precipitated out. That's why you could see it.

10 ppm FC with zero CYA is a very high FC.

If you had added the CYA and then the FC it probably would not have precipitated the iron and you'd have never seen it. So, doing what you did may have actually helped you a little as long as it didn't leave any more stains than you previously had. In fact it may help the stains you have now. It allowed you to filter out a little iron which is always a good thing.
 

Mandy

Silver Supporter
Jul 5, 2017
34
Dubbo Australia
Thank you for your reply. As expected my water is back to beautiful and yes, how lucky was I! Being able to filter iron out is such a bonus as opposed to it attaching to the pool surface. I’d love to know why that is.......could the secret to forcing this to happen lie in having zero CYA? I’m currently adding my second dose of CYA and will be monitoring the water closely in the coming days.
 

Bama Rambler

Mod Squad
TFP Expert
Bronze Supporter
LifeTime Supporter
Jun 22, 2009
23,873
SouthWest Alabama
Precipitating iron by raising FC is hit or miss. Once in a while it works, but most of the time it doesn't. I'm sure Matt @JoyfulNoise can give you the chemical process of how that happens.

Actually in your case the low CYA was a contributing factor, not because it was zero, but because you raised your FC so high in relation to the almost non-existent CYA. It's the high FC that causes the conversion of the iron.
 

borjis

LifeTime Supporter
Aug 19, 2014
3,100
Pacific NW
I have this issue every other year. After adding the first bleach of the season. The water report from the city shows traces of iron but not much.

It's hot or miss for sure. My liner has a uniform barely visible tan discoloration as a result. Gonna try the polyfill & jack magic pink stuff when it shows up next time. Previous years I'd just replace half water & it goes back to blue. Can't explain why and it this point don't care. Just keep it a nice blue :)