Hello, do any of you give your cats vitamins? If so, what kind and in what form? Harry is prone to struvite crystals and hairballs. Plus, he's going on 7 years old, so I want to make sure he stays healthy and happy! I also have Maggie, who's only 1 1/2, but they share food and water bowls ... so whatever Harry gets, Maggie gets! Therefore the vitamins I choose should be good for both furbabies. Thanks in advance for your information!