Shock or MPS is not a sanitizer like chlorine. That being said you can get a shock/chlorine mix like replenish. If your using that type of a product you don't need to shock. If you are using chlorine and it says on the btl 99% sodium dichlor then you need to shock with MPS at least once a week.. Some will use MPS (shock) after use, makes no sense to me to add it an hour before you use it, then chlorinate once or twice a week. It's not always about the size of the spa, but how many are using it..