My cat has had struvite crystals.  She was doing very well, and was
finally taken off medication and could stop eating c/d prescription.
She went from eating c/d (with chicken) to eating Innova and in just
two weeks, the pH of her urine went sky high so she's back on c/d.  The
vet thinks Kitty is very sensitive to fish (Innova has fish) and
magnesium.  I had our cat sitter ask about other Science Diet foods
which have low magnesium levels, and the vet suggested Kitty stay on
c/d permanently, since c/d has certain medicines or other ingredients
that help it keep the pH level of urine in an ideal range.

My research on the SD site says c/d does have added antioxidants, but
the magnesium, phosphorus, etc. levels are very similar to other SD
foods, and there are other SD foods that are designed to keep the urine
at a 6.2-6.4 pH (same as c/d).

Do antioxidants really make that much of a difference in controlling
crystals and the pH level of urine?

I'm thinking of alternating between c/d and another SD food (SD Gourmet
Turkey Entree, Savory Chicken Entree, Beef or Chicken in Gravy Savoury
Cuts), all of which are designed to keep the urine at 6.2-6.4.  But I
have some concerns:

Some of these do have fish (ocean fish or salmon) as one of the
ingredients.  Is that bad for my cat, who is supposedly sensitive to
fish?

The magnesium levels are about the same as c/d (the savoury cuts ones
are even lower in mag, while the gourmet turkey is at .065 compared
with c/d .058).  How much of a difference in magnesium will affect her
health?

The phosporus levels are higher than c/d, though (most are in the .7
range, with the highest at .8 with c/d being .66.  How much of a
difference in phosphorus will affect her health?

Are there any other levels I should be concerned with when dealing with
struvite crystals?

The SD website warns against feeding other foods with c/d, because the
efficacy of c/d will be affected.  But since Kitty is on maintenance,
would it be OK to feed her c/d AND other foods?  (SD in the morning,
c/d at night, for example)

I researched here that others have switched their cats from c/d to
other foods (Science Diet or other) with success, but I am a little
worried because Kitty has proven very sensitive to fish/magnesium.  c/d
is quite expensive for me, and if I have to keep her on c/d, I will.
But if I can switch her to something a little more affordable for me,
that would be great.