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I have a swimming pool in Portugal with a heating system of aluminium (good heat conductor) tubes, painted black and a small circulation pump. The sun shines, the pool gets warm, cost is minimal and it works well.

As the pool is private, I use a minimal level of chlorination and therefore I use a small amount of copper sulphate (<1 ppm) to control algae growth.

I notice that the heating circuit inlet pipe to the pool leaves white crystals on the floor of the pool.

Question - am I correct in thinking that the copper sulphate is reacting with the aluminium tubes, resulting in a copper coating on the inside of the tubes (not that I can see inside) and aluminium sulphate crystals appearing in the pool?

I understand that aluminium sulphate is essentially 'flocculant' and is not particularly hazardous: certainly the pool stays very clear.

Advice would be much appreciated, many thanks

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    $\begingroup$ There is the standard redox reaction, as I show here, but you have a very dilute solution of copper sulfate, and dilute aluminum sulfate is harmless. I would not worry about it unless the situation worsens. In that case, use a different, non-corrosive, algaecide. $\endgroup$
    – Ed V
    May 23, 2022 at 18:31
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    $\begingroup$ Your white crystals are not aluminum sulfate, because aluminum sulfate is soluble in water. It may be calcium sulfate produced by the calcium ions dissolved in the tap water after reaction with the sulfate ions from copper sulfate. Or it is calcium carbonate produced by heating the tap water. Because soluble calcium bicarbonate from tap water is decomposed in hot water into insoluble calcium carbonate, which produces nice white crystals. This deposit is often a problem in washing machines. $\endgroup$
    – Maurice
    May 23, 2022 at 21:49
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    $\begingroup$ Thank you Gentlemen, The tap water is soft here - negligible scale. I doubt there is any danger from whatever it is, no skin irritation or anything like that, however, out of curiosity I think I shall take samples of water and crystals to the local lab. for analysis. I'll post the results here for your interest. $\endgroup$
    – Michael Candlin-Pass
    May 24, 2022 at 10:37

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