TCTrun, stone formation in guinea pigs is poorly understood. But there is considerable evidence that calcium in the water does affect it, and therefore giving hard water to pigs is not recommended.
For pigs who have powdery or sludgy urine, it is also not fine to feed romaine lettuce if that is the cause of the powder or sludge. Some pigs eat romaine all their lives without problems, other pigs cannot.
Your incorrect advice may result in someone having pigs with urinary tract stones, causing pain and suffering for the pigs and expensive vet bills for the owner. Please do not post advice that runs contrary to generally accepted practice unless you specify that it is your opinion only, and that it does not agree with other peoples' experience with their pigs.
Squint96, you do need to worry about the calcium in your water. Cavy experts in England routinely recommend that pig owners in the south of the country (where the water is very hard) filter their water. You can get an idea of whether water is soft or hard by how much soap suds it creates. Put some warm water in the sink, add a little dish detergent, and swish your hand around in it. If it makes lots of suds which last for a while, you've got soft water. If it doesn't suds much, it's probably hard. Another way to tell is by how hard it is to rinse shampoo out of your hair -- if it comes out easily, the water is hard. If you have to rinse forever, it's soft.
If yours is hard, a Brita filter on your tap, or a Brita pitcher will do the job just fine. Or, if you have a newer refrigerator, there may be a filter on the water line. Just be sure to change the filters on the recommended schedule.