Karri -- I have a ferret who not only had crystals, but they formed into
bladder stones several times during her life.  We also tried the special
diets, and were told that she would need to be on them the rest of her life
(this was almost five years ago).  However, she ate barely enough to stay
alive and refused the other food supplements we were trying to give her, so
that idea collapsed.
 
What we had to end up doing was putting her on medication to control her
urine pH, and she's been on it almost 4 1/2 years now.  If I stopped her
medicine today, there's no doubt she'd develop stones again.
 
I don't know what *type* of crystals you have, but I'm not sure 2-3 weeks on
a special diet will treat the problem.  Generally crystals form because in
general a ferret has too much/too little (I can't remember which) acid in
the urine.  I would see what the ferret specialist says, but if you need any
more information, feel free to email me and I can give you more specifics on
Claudia's condition and how we've treated it.
 
Amy & Dave (and their gaggle of giggling ferrets) RIP Ian and Elektra
[Posted in FML issue 1811]