How do I stop my mask from fogging?

New masks carry a manufacturing film — scrub the inner glass (toothpaste or a burn-in per the maker’s instructions) or it will fog forever. After that, a drop of defog or baby shampoo, rinsed once, before every dive. Persistent fog usually means the film survived or the skirt is leaking a trickle. Our defog guide ranks the treatments that actually last a two-tank day.