Can you describe the little beige bug you found? It doesn't sound like an insect that would infest your tanks. Most likely it went in there while the cover was open and you were doing something with your snake.
I wouldn't recommend you treat your snakes with Nix unless you have mites or suspect them. It is a good safe treatment but I wouldn't use it if it wasn't necessary.
Keeper of 5 snakes, leopard geckos, 1 green iguana, 20+ tarantulas, 2 dogs & a bunch of rats!