A fogged up mask is always an issue for scuba divers and snorkelers alike. No matter what defog you decide you still need to prepare your mask before using it. High quality masks when they are made need to be prepared by scrubbing the lens with a mild abrasive like baking soda or tooth paste. What this does is remove a surfactant caused by the silicone skirt secreting a release agent on the lens that cause them to fog up. The release agent is sprayed into the moulds of the skirt where the silicone is pumped into. This release agent secrets on the lens and need to be removed by the mild abrasive. Once this is done a good quality mask defog is key to maintain mask clarity.