Velvety Cookies

Let your medium get really dry and then cover it with a good layer of sand like you suggested. I wouldn't bother with DE as if it gets wet it won't do anything until its dry again. If you wanna kill em fast you can do what you already had in mind and get mosquito bits/dunks and soak em in your water overnight. They are made with a type of beneficial bacteria that kills fungus gnat larvae. That 2 part plan should knock em out for good in a couple days. I used mosquito bits and already had gnat nix in place(didn't really seem to do anything so don't waste the money) and added some extra airflow over my pots and they were gone in 2 days.