Diet Calcium Pee Stains

My boys- mostly Benjamin at least has gritty white pee stains again. He is currently on medication for a UTI with possible stones 9 we're waiting to see if the squeaking comes back after the medication is through). Anyways they usually eat for veggies green leaf lettuce, red, yellow, orange and green peppers, cilantro and a carrot. Are there any vegetables I can give them to help them? They have sweet meadow pellets and hay (both timothy). Are there any changes I should make to this diet? They currently drink water from the tap (as does my whole family) but I'm going to change them back over to baby water that is purified.