Could you post what the label shows is in the 4 in 1?
I suspect that something in it is driving your pH down.
About your CC: You probably are fighting the beginning of an algae bloom, and will need to add plain household (unscented, etc) bleach to bring your FC level up to ~15, ( Bens best guess chart )and keep it there until you lose <1 FC overnight, and CC remains at 0.
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