Once was thought that magnesium in the fish was causing struvite, but new research shows this is not the case. Magnesium chloride acidifies the urine, and magnesium oxide makes a more alkaline urine. There are 2 different kinds of crystals here struvite and oxalate.
Salmon and some other cleaner fish is not as high in magnesium as Tuna, it is alot higher.
*I will only feed my cats salmon once a week, they do not get Tuna at all, here are some of the reasons why I dont feed Tuna (on a daily basis.)
-Makes for a picky cat ( cats need a variety in diet)
-high in mineral salts (this is what is bad for the cats urine)
-high in polyunsaturated fats, which cats cannot metabolize.
-Tuna in vegie oil takes vitamin E away from the cat.