Purple Deposits and stains

[skimberley]

I have a fiberglass inground pool, 7m x 3.5m (23ft x 11.5ft), 25000 liters (6600 US gallons). On starting up for the summer the pH was very low, so I over a few days I added a few kilos (several pounds) of Soda Ash (sodium carbonate) to raise the pH to the correct range. A few days later, purple deposits started appearing on the sides and bottom of the pool, which increased over following days. The purple material is has a fine, slightly gritty texture, and while the excess material vacuums off, some of the material sticks to the surface. Brushing removes it partially, but there still remains a purple'ish residue, and I have to clean the pool every few days.
For my Chlorine I use multi-action tablets (pucks?) which are based on Tri-chlor. The water is perfectly clear. I am located in Cyprus (East Med), and the day temperature is currently 30-33C (86-91F).
Any advice please on what is the cause of this and how to get rid of it? Thanks

 

[Texas Splash]

Sometimes when someone says they notice dark stains appearing once the pH increases it can be metal related (iron or copper). Do you know if you have any such metals in your local water? Do you use a copper-based algaecide perhaps? On a well? But those metal stains will not seem gritty or brush off, so that is odd. A couple other questions that might help us get started:
- A full set of water test results would be key, but from your intro post we know you are using test strips, so that is a limiting factor at the moment.
- Update your signature with all of your pool and equipment info. That may help.
- If you've added any other unusual products to the water let us know.
- Do a test by rubbing a chlorine puck on a stained area, but keep it moving slowly - not still. Also try rubbing a Vitamin C tablet on the stain.

We'll do what we can to help you.

 

[skimberley]

Thanks for your comments above, sorry for delay, I have been away a few days.
My pool is a fiberglass inground pool, 7m x 3.5m (23ft x 11.5ft), 25000 liters (6600 US gallons). My pump is "Calpeda" (Italian) with a 15m3 (15000liters or around 4000gallons)/hour capacity. The Chlorine pucks I use contain 90% Tri-clor, 3-5% Aluminum Sulphate + 2-3% Copper Sulphate, + 1-2% Boric Acid. My Algicide (rarely used) is 7% Quaternary Ammonium.
My current readings as best as I can tell from the test strips are:
Hardness 500
Total Cl (test colour does not match the chart!)
Total Br (test colour does not match the chart!)
Free Cl 1 (but could be up to 3, its hard to match the colours)
pH 7.2 (but could be 7.8)
TA 120
CYA 100 (could be 150!)

I did add some algicide a couple of weeks before raising the pH.
The city water here has a high pH, around 8.2, but oddly enough, my pool pH tends to be low. I very rarely have to reduce pH, and more often have to raise it.
Rubbing a tri-chlor puck onto a stained area has no noticeable effect.

Thanks

 

[JamesW]

Probably copper cyanurate from high copper levels and high cyanuric acid levels.

https://www.poolspanews.com/how-to/maintenance/when-pools-turn-purple_o

https://aquamagazine.com/features/avoid-opening-pools-to-copper-stains.html

 

[Texas Splash]

Your situation is going to be a tough one for us I'm afraid. Some TFP members around the globe simply don't have access to all the chemicals best-suited for residential use. We know test strips are simply horrible for pool owners because they are so unreliable. So based on your supply sources and ability to have items shipped to you, this is what I would recommend:
- See if you can get a TF-100 (link below) or Taylor K-2006C test kit mailed to you. Also check the link below to see if that helps. A proper drop-test kit is a must.
- As best as you can right now, control the pH and keep it on the lower end of the 7 scale. The higher the pH gets, the more stains try to appear.
- Look for a source that has liquid chlorine or regular bleach - same thing. Avoid the other products if you can.
- Algaecides only prevent algae (sometimes) but never kill algae once it has started. Also remember that some algaecides contain copper and you don't want any copper in a FG pool (stains).
- If your CYA is truly close to 100 (from those tablets), a water exchange will be needed first to lower it. Then start the liquid chlorine (no CYA in it).

Pool Test Kits - Further Reading

www.troublefreepool.com

 

[skimberley]

Your situation is going to be a tough one for us I'm afraid. Some TFP members around the globe simply don't have access to all the chemicals best-suited for residential use. We know test strips are simply horrible for pool owners because they are so unreliable. So based on your supply sources and ability to have items shipped to you, this is what I would recommend:
- See if you can get a TF-100 (link below) or Taylor K-2006C test kit mailed to you. Also check the link below to see if that helps. A proper drop-test kit is a must.
- As best as you can right now, control the pH and keep it on the lower end of the 7 scale. The higher the pH gets, the more stains try to appear.
- Look for a source that has liquid chlorine or regular bleach - same thing. Avoid the other products if you can.
- Algaecides only prevent algae (sometimes) but never kill algae once it has started. Also remember that some algaecides contain copper and you don't want any copper in a FG pool (stains).
- If your CYA is truly close to 100 (from those tablets), a water exchange will be needed first to lower it. Then start the liquid chlorine (no CYA in it).

Pool Test Kits - Further Reading

www.troublefreepool.com

Thanks, very helpful.

 

[skimberley]

Your situation is going to be a tough one for us I'm afraid. Some TFP members around the globe simply don't have access to all the chemicals best-suited for residential use. We know test strips are simply horrible for pool owners because they are so unreliable. So based on your supply sources and ability to have items shipped to you, this is what I would recommend:
- See if you can get a TF-100 (link below) or Taylor K-2006C test kit mailed to you. Also check the link below to see if that helps. A proper drop-test kit is a must.
- As best as you can right now, control the pH and keep it on the lower end of the 7 scale. The higher the pH gets, the more stains try to appear.
- Look for a source that has liquid chlorine or regular bleach - same thing. Avoid the other products if you can.
- Algaecides only prevent algae (sometimes) but never kill algae once it has started. Also remember that some algaecides contain copper and you don't want any copper in a FG pool (stains).
- If your CYA is truly close to 100 (from those tablets), a water exchange will be needed first to lower it. Then start the liquid chlorine (no CYA in it).

Pool Test Kits - Further Reading

www.troublefreepool.com

I note your advice about using liquid chlorine, which I presume is because it is unstabilised, and therefore does not contain CYA. Liquid Chlorine is not available from pool suppliers in Cyprus, except as household bleach from a supermarket, and I presume this would be not of a high eenough concentration. The only forms of Chlorine available for pool use are DiChlor and Tri-chlor, either in granular or multi action tablet form, both of which are stabilised and presumably therefore contain CYA. My question is if I use any form of unstabilised Chlorine, will it not just evaporate the next day? I thought the whole point of stabilised chlorine is to prevent it evaporating quickly, and if using unstabilised chlorine, then would I have to add it every day?

 

[mknauss]

Once you have sufficient CYA in the pool water (30-70 ppm depending on conditions) you do not need to add any more. That is the issue with dichlor and trichlor. Those products continue to add CYA. The chlorine still is used by the UV from the sun and organics, and must be replaced every day.
If you are only using dichlor or trichlor, a 1/3 or so drain of the pool volume every 4-6 weeks will keep your CYA at least in a manageable range.