The most important item to sanitize the tub is sanitizer!
The silver cartridge whether Nature 2 or the Frog is a suppliment to the sanitizer.
Personally I don't use a silver cartridge in my tub, just use chlorine. If you add chlorine and it reads 3 PPM 20 minutes later - you've done good. Adding more gives an extra protection to the water that doesn't hurt anything. My belief is I can buy 5 lbs of chlorine for the price of 1 cartridge so I buy the chlorine and it lasts more than the 3 to 4 months that the cartridge would.
You really don't need any cartridge.