Magnesium, staining and calcium

Been managing my pool succesfully since I inherited it 6 months ago.
Fibreglass, 50,000L, SWG salt water with sand filter.

Last weekend I did my monthly tests and got these results:
Free chlorine: 1.5
Combined chlorine: 0
pH: 7.5
Total alkalinity: 60
Calcium hardness: 225
CYA: 100
Salt: 4700
Temp: 15 degrees

I went down to the hardware store and got 6 litres (200 fl oz) of 4% bleach, 7kg (15.5lb) of calcium chloride, 100g (0.2lb) of soda ash and 12kg (26.5lb) of magnesium salt; as per the...

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