Hard water will stain light colours
We have both bore water (hard) and rainwater (soft) here and I never wash the light load or nappies in the bore water. Made that mistake with some towels that are forever off white now!
BUT...I would imagine the reason for holding the poo stains is that you need WAY more detergent for hard water.
Not sure about Calgon?
Sorry for being brief, bub needs to go to bed! xx