My pool water turned cyrstal green colour a couple of days back after increasing my TA & pH, water samples were drawn from pool and back wash and was tested for both iron and copper.

(1)Pool water showed no signs of copper OR iron.
(2)Backwash water showed a slight level of iron.

I then added bleach into both water samples (1) pool water remained clear (2) backwash water turned yellow.

So i then went to reduce my pH using Hydrochloric Acid and double dosed with an HEDP based sequestrant only to find that the water has gone from light green to a darker shade of green.

I have read through some comments and heard that adding hydrochloric acid to water that contained copper could cause water to turn into a darker shade green is this true?

Would it also be possible that the trace metals got stained onto collidial particles as one article i read commented? If so should i use a floc/clarifier to treat this problem?

If not what can i do to treat my pool?

My test results as follows:
Cl: 3.0ppm
pH:7.5
TA: 60
Cal Hardness: 90ppm
Temp: 27C
Iron: 0ppm
Cu: 0ppm

BTW during my hydrochloric acid treatment i have continued to backwash at least 3 -4 times a day for the pass few days.

I know of two different causes for green water.....

1. Sudden introduction of chlorine into pool with high iron content

2. Algae

Have you completely dismissed algae as the problem?

What is your CYA? 3ppm FC may not be enough if you have a high CYA level...

What is your usual chlorine source? pucks, bleach?

100% sure it has nothing to do with algae.
CYA 25ppm.

Usual source of chlorine for me is a mixture of 12% bleach & TCCA.

Then, unless someone can come up with another plausible answer, you have high iron content in your water and introduced a fairly high level of chlorine, causing the iron to precipitate out.

In this case why has the colour of water darkened after adding HCL and some HEDP?

What bothers me is how could your pool water sample show no iron but your backwash sample shows iron?? It's the same water except the backwash water is flowing backward through the filter media. Unless your filter or media is rusting inside and you broke some loose backwashing there's no reason for it.

How did you make 100% sure it's not algae? What process did you use?

The water from my filter goes through some old metal piping when i backwash, which is where my sampling point is and as such i believe iron content tested would have been coming from the pipe.

As with how i am so sure its no algae:
(1) Water itself until now is clear but tinted dark green, unlike murky green you get with algae.
(2) Overnight FCL drop test was done.
(3) TCL was tested for as well.

Can you experts please give me some advice /help?

Thks

How are you testing for FC...ie which test kit? Also, what is your CC level?

Unless the pH was way out of line, I don't really think your backwash water is going to pick up a measurable amount of iron from being run through metal pipe. Certainly not enough to cause the water to discolor upon adding bleach.

Is your pool surface blue? Often, yellow stained water looks green in a blue pool. Try filling a glass jar with some water and looking through it at a white surface to verify the color if your pool is blue.

There are different types of algae, and some do not give the murky water common to green algae.

Test Kit: Trouble free test kit (FAS DPD).
CC @ 0.5ppm

Pool Surface is white, will try and look at water through glass again today, but as i recall water looked clear with a very slight turbid feel to it.

Hmm i added bicarb and next day i find my pool water green.....

WIll retest today and pst full set of results.

Not trying to antagonize you but.......

I know of only two different causes for green water.....

1. Sudden introduction of chlorine into pool with high iron content

2. Algae

A high pH and TA could precipitate copper carbonate which is usually a light green though if more intense it can look darker green, but still "clear". Iron oxide-hydroxide is the other possibility and is usually more yellow in color (which on a blue background looks green).

If you had metals and they mostly precipitated, then you might not register free metal in the water. What is strange is how lowering the pH caused the pool water to get darker green since normally lowering the pH would redissolve some of the precipitate and lighten up the water. Normally with a metals problem you would lower the pH, add a metal sequestrant, then slowly raise the pH to normal levels. Water replacement with water free of metals is the only long-term solution. Metal sequestrant breaks down over time so would require reapplication periodically.

Test Results (Nov 25h, 09)
FCl: 2.8ppm
TCl: 2.8ppm
CYA: 25ppm

IRON: 0.4ppm
CU: 0ppm

pH: 7.4
TA: 50ppm
CAL: 40ppm

Added more HCL, and PAC started to see an imporvement in the water quality. Backwashed filter and found alot of brown coloured water coming out of filter.

Chemgeek i thought if metals precipitated it should stain? Whereas if metals were held in water it should cause the pool water to discolour and that the metals would have been able to be tested by my kit?

When metals precipitate, they don't necessarily stain. They can come out as a suspension coloring the water instead of staining pool surfaces. I don't understand why, but suspect it has to do with the specifics of the pool surfaces themselves and their tendency to have metal percipitate stick to them. It may also just take time. Generally speaking, standard plaster tends to get stained while vinyl doesn't as much. Again, I'm not sure what it is about your particular pool surface that didn't stain, but consider yourself lucky.

That brown color is usually caused by dead algae but it also could be the iron that precipitated out into suspension in your water and then was trapped by the filter.

If held in suspension should i not be able to test for the metal contents? Why was that i tested negative for metals first few days? Dont understand .

BTW water has turned from Brown back to Blue.

Thks Guys.

When the metal precipitates, it will no longer be measured in metals tests because the metal will be in compound form that is a solid, though could be so fine a particulate that it is suspended in the water and reflects light to color the water. Metal tests only check for metal ions -- that is, metal that is dissolved in the water, not solids that are in the water. That's why your metal tests didn't measure much after the color fully formed. This also explains, as Dave said, why your backwash water was so dirty -- your filter was probably filtering out a lot of the metal compound solids. When you lowered the pH, you essentially redissolved the metal compounds back into metal ions that not only will measure in a metal test but can be sequestered by a metal sequestrant (in the case of iron, this may require an ascorbic acid treatment since the iron must be in ferrous form to be sequestered while chlorine can oxidize it to ferric form that won't be sequestered).

I suppose that one could take an approach of keeping the pH raised and try and filter out the metal, possibly using a clarifier to help, but that is risky since the metal could stain surfaces during that time.