Don't use Nix! Even diluted can harm the snake! If you want to kill mites:

Give snakes clear water bath, then heavily coat them in mineral oil and let them hang out in a tub for about 15 minutes. The oil smothers the mites. Rinse the snake off as well as possible--he'll still be oily for 2-3 days--in a betadine solution bath. The mixture should look like weak tea. You also need to clean the tank, especially under the lip of the top and around the seams. Best bet is to fill the tanks completely with water (outside, with a hose) and let it sit for a while. You can also use Flea shampoo for cats and dogs in the tanks.
Lastly, bug bomb the room since mites will jump out and hang on to curtains and in carpets. You need to take care of the tank and room if you want the mites gone, not just the snake. Using mineral oil is MUCH safer.
We had an infestation and four snakes at the time and we did these things in one day and by the next the mites were gone and we've never had them since.

Good luck. PLEASE don't use Nix on your snakes. Even a diluted mixture can kill them!