Why is there a loss of grouting in mosaic/tiled pools, or sharp edges around tiles?

There are two possible factors at work here:

• In soft water areas, the grouting is being etched by the pool water. This is because there is insufficient calcium in the water. There is a tendency for water to form an equilibrium by searching for calcium – in this case from the grouting or plaster substrate.
  • Regrout the pool and increase calcium levels in the water by adding calcium chloride  so as to achieve a minimum calcium hardness level of 250ppm. Alternatively, use calcium hypochlorite for shock dosing or for regular sanitisation – calcium will be automatically added to the water in using this sanitiser.
• High levels of sulphate in the water. You will need to get your pool profesional to test for sulphates to confirm this diagnosis.
  High sulphates are caused by (a) high sulphates in the mains water (b) the frequent use of dry acid (sodium bisulphate) or (c) use of aluminium sulphate as a water clarifier.
  • If you suspect factors (b) or (c), dilute with fresh water and switch to alternative methods of lowering the pH (e.g.using trichlor as the main pool sanitizer), or clarifying the water (a sulphate-free treatment). There is not much you can do about (a).