Rabbits and hard water
I put this together earlier this year for something else, but it may be useful here. Just to put the whole hard water issue into context:
Moderately hard : 60-120 ppm (parts per million) or mg per litre (as CaCO3)
Hard water : 120-180 ppm
Very hard water: over 180ppm
Where hardness is calcium and magnesium dissolved in water
Rabbit optimum dietary level of calcium required = 0.6-1% (referenced from FHB)
Average adult 2.5kg rabbit needs 500mg calcium per day
Equivalent dietary intake: 113g hay or 152g dandelions or 157g spring greens or 463g grass or 5 litres of very hard London tap water
Twickenham vets tap water: Calcium carbonate(CaCO3): 260 ppm
Ca:CaCO3 molecular weight
40:100
Therefore calcium = 104mg / litre
So 0.25 litre is 5% of recommended daily intake of calcium for an 'average' rabbit in a very hard water area
Rabbit metabolism means that excess calcium is excreted in the urine. FHB recommends that increasing fluid throughput (ie drinking more) and being more active will reduce the likelihood of bladder sludge. The calcium in water is only a very small part of the overall dietary intake
https://www.water-research.net/index.php/water-treatment/tools/hard-water-hardness
https://en.wikipedia.org/wiki/Hard_water
https://www.harcourt-brown.co.uk/articles/free-food-for-rabbits/calcium-and-rabbit-food
https://www.thameswater.co.uk/Help-.../Check-the-water-quality-in-your-area/success 02April2018
You can find out the hardness / calcium content of your own tap water from your supplier - it is usually published online, but you can always ring and ask as sometimes it depends on the exact supply to your specific zone within their supply area. Basically, your rabbit would have to drink an awful lot of very hard water to make much of an impact on dietary intake of calcium, compared to eg. a few pellets.