Purple water in swimming pooĺ

[laser]

The likely underlying cause is the too high concentration of cyanuric acid. Have you checked its level? While chlorine is continually consumed in the water, cyanuric acid is accumulating and can reach high levels over time. Like explained above, this chemical can form pink/purple compounds with some heavy metals (magnesium is not that and not involved) that are often present in pool water at low concentrations.

[Dieter78]

What kind of test kit do you have?

Here is a snip from pool wizard:

"Total Alkalinity (re corrosion & staining floors).

The total alkalinity (TA) is a measure of how much of the alkaline substances there are in the water. In the swimming pool water, we are concerned with bicarbonate alkalinity, which should be between 80 ppm and 120 ppm.

When the total alkalinity (TA) is within this range, it prevents rapid pH changes and "stabilises" the pH level.

If the TA is too low, Marbelite and plaster walls will become etched, metals corrode, the pool's walls and floor can stain, the water can turn green, eyes burn and we can have pH bounce (pH rapidly going up and down, seemingly at random)."

I use these strips to test my water...:

[Dannie Boy]

Are these strips available at the local pool shops and rough idea on price?