Angular’ stains are distinct from the smears or sheets associated with stains caused by water chemistry because they are caused by chemical reactions BEHIND the pool surface.
- Poor Construction/workmanship. Marbelite may be too thin in places/Gunite applied to thinly in places, resulting in oxide leaching from rusting metal reinforcing.
- ‘Calcium Hardness’ too low for too long, OR pool to acidic for too long resulting in calcium leaching from the Marbelite, making it weaker/porous.
- Age of the shell
Consult a reputable pool renovator.