My cat was showing signs of a bladder infection yesterday, my normal
vet is on vacation, so I took Fluffy to a new vet we'd never seen
before. Anyway the urine analysis showed a bladder infection with some
crystals. Apparently they weren't very many, but they are quite large.
They gave him some subq fluids then sent me home with a bag of Hills
c/d and said I should watch to ensure that he's able to urinate, if
not then to take him into an emergency vet tomorrow or Sunday.

I've never had a cat with this problem before, so is just a dietary
change normal? I kinda expected that he'd get some antibiotics or
other medication to help clear things up.

What causes this type of problem? Fluffy eats innova dry and wellness
canned food, so I thought I was doing pretty well. Looking at the
ingredients list for the C/D it frankly looks to have pretty poor
ingredients.