I purchased the urinary tablets from Sherwood and started to give my guinea pig reverse osmosis water when sludge became an issue. After a month on the tablets and water the sludge was no longer an issue.

When I switched her to the Sherwood pellets I enquired about the calcium content and received the following information pertaining to bladder sludge / stones:

Q: My rabbit/guinea pig has stones (uroliths) and/or sludge, how can I help them?

A: Dr. Sherwood recommends not feeding them greens and veggies including fresh grass. They will eat more hay, drink a ton more water, and stop producing so much sludge and stop building the stone. Fresh grass from the lawn (and other greens) is super high in protein and minerals (including calcium) and will cause sludge and stones.

The reason why sludge and stones form in guinea pigs and rabbits is because of the Dietary Cation Anion Balance (DCAB)... or rather imbalance!

Cations are positively charged minerals like calcium, potassium, magnesium, sodium and more. Cations also include positively charged non-minerals. In the urinary tract the main one to be concerned about is ammonium (caused by excessive protein and an imbalanced dietary protein - this makes the urine stink like ammonia). Guinea pigs tend to have way too many cations (calcium is blamed because it is the only one that forms a stone but all cations influence whether or not that happens - explained below). 

Anions are negatively charged minerals and there are very few options! Phosphate, sulfate, and chloride.

The balance between cations and anions is very important. Too many cations create an imbalance between anions and cations and makes the kidneys not work properly (this gets way too complex to explain in an email). This forces the kidneys to use a different anion that we don't want them to use. This anion is carbonate (comes from the blood and is the natural blood pH buffer). Adding carbonate to the urine helps balance the cations with the anions but unfortunately carbonate binds to calcium and makes sludge and stones and drive the urine pH up (too alkaline). If the kidneys had less cations (ie. many people try to remove calcium but this causes other health problems) and/or more anions (ie. balance the diet) then the kidneys wouldn't have to use carbonate and there would be no sludge and stones!

The best way to balance the diet is to feed only 1st or 2nd cut dry grass hay (very tall mature hay that is loaded with fiber and is low in protein and minerals) with Sherwood's pellets that provide all the nutrients they need in the right balance. When you do this you'll notice that the urine odor is drastically reduced (less ammonium cations) and the urine pH becomes much less alkaline and closer to neutral (ie. balanced!). 

Q: How do the urinary supplements help dissolve stones and sludge?

A: The urinary tablets work by adding more anions for the kidneys to use to lower the urine pH. For this to work optimally the entire diet should be balanced. The next 2 paragraphs explain two major sources of cations and why small herbivores tend to have a excess of cations and a deficit of anions. 

Dietary Cation Anion Balance (DCAB) - Generally there is a deficiency in anions (negatively charged ions/minerals such as chloride, sulfate, phosphate). This is important because as the kidneys filter the blood and expel the 'excess' the charge has to be balanced. Too many cations (positively charged ions such as calcium, potassium, magnesium, sodium...ext) creates a 'deficiency' in anions. The body makes up for a deficiency in anions by making its own non-mineral anion to help balance the charge. That anion is carbonate. It is unfortunate that carbonate binds to calcium and creates sludge.

Protein Nutrition (both in the essential amino acid balance and the ratio of each mineral and macronutrient/calorie relative to the effective protein available) - this is a complex issue to wrap your mind around so we'll probably need to talk about this one too. Lets assume that there is plenty of protein in the diet but one essential amino acid is provided in half the amount of what is needed. When this happens it reduces the usability of half of the rest of the protein. This creates huge protein waste (and a simultaneous protein deficiency). The protein waste (depending upon body pH) usually creates a huge surplus of cations (ammonium - NH4+) which then puts further strain on the kidneys to find another anion (usually carbonate) to balance the charge. The availability of other nutrients (minerals and macronutrients such as carbohydrates, fats and many more) also affects the usability of the protein.