filtered

I'm looking at getting a water filter for the pigs as Toast has recurring bladder problems, cystitis, UTI, bladder crystals etc.. I noticed some are over £100, some at £20 and some are cheap, around £5.99... I've also read that some filters do different things and filter out more/less...

Lately I have been hearing that filtered water is better for pigs because it doesn't have as much calcium as tap. I've been using tap water ever since I got the pigs, and they are now 8-10 months old. Now that they are older than six months, I think it would be better if I gave them filtered...