This is taken from that article written by the vet that I posted – it’s an excellent article and you should all read it because bladder sludge is more complex than just the veggies you give your rabbits and it explains it very well:
Feed large amounts of fresh leafy greens – Fresh leafy greens are not only an excellent source of fluid for the rabbit (to keep the urine dilute) but also contain a variety of nutrients (including vitamin C) and fiber (for the digestive tract). Use dark leafy greens that are richer in nutrients than the pale green types. Feed a minimum of three different types of greens daily, don’t stick to just one type. Examples of greens to feed include kale, mustard greens, dandelion greens, parsley, cilantro, chard, beet tops, carrot tops, raspberry leaves, broccoli leaves, and Romaine lettuce
As mentioned previously, although the calcium content of dark leafy greens is higher than some other foods, it is not all digestible and doesn’t approach the level of commercial pellets or alfalfa hay. NEVER attempt to remove all calcium sources from a rabbit’s diet or problems with abnormally low calcium blood levels, such as bone, dental and muscle disease can occur.
We do not consider feeding dark leafy greens a part of the problem causing urinary calculi and in fact we consider these foods to be an essential part of a healthy rabbit diet.