Blue stain is back.
We have had a lot of rain recently and the tanks are full with about 30 kL each.
Tanks read an uncalibrated pH of 5.7 as measured at the kitchen tap.
By uncalibrated I mean that the pH meter has not been used for a while and I should calibrate it before use - but I didn't.
I treated one tank with 500ml lime and instantly it rose to 9.2.