Since we lost Willow to stones, Bracken's stone op, Spike and Caspy's sludge I'm finding it increasingly difficult to feed the piggies. They used to be eat a lot of grass when they could go outside but then got giardia and worms from the grass. I've lost my confidence really that I'm doing the right thing at all. My vet gave me a list of what to avoid, occasional treats only and good choices when trying to reduce dietry calcium. Too much to put all the lists here but the good list is Broccoli (not leaf), Cilantro, Dark leaf lettuce, Watercress, Celery (leaves only), Cabbage, spring greens, endive, pepper, fine green beans, cucumber. I get a lot of sludgy deposits on the fleece feeding any of this except the last three, lettuce and spring greens. I really feel like

, when looking at these lists the only bad thing they'd had was occasional sow thistle and carrot tops but not enough to cause the problem. Pellets are limited, water is double filtered, on paper these piggies had had just about the best stone avoidance diet possible and yet we have 4 out of 6 piggys with problems.
Some abreviated notes from my vet.
It's not just calcium carbonate that's the problem either, there's oxelates to consider, (spinach, celery, parsley, strawberry are high). Too much vit C can help the formation of stones due to oxelate levels. With supplementation limited to 25-100mg/kg per day it's unlikely to result in excess oxelate levels.
She also stressed that diuresis is extrememly important and fluid intake needs to be increased to flush the bladder. Bracken is being syringed 10ml water 2x daily since his op. The other 2 sludge formers get 10-15ml twice a week to flush out any sludge. I've witnessed this work with Spike, I'd just given him 15ml and returned him to a pale green blanket on the floor he squeaked and weed 4 lumps that looked like cream cheese and 2 spots of blood. This was the first time I did it and he's not had a recurrance that bad since but does still wee thick sludge sometimes rather than just white wee.