I have never reused substrate, I switched tanks because of the fly problem. The old tank is wrapped over with plastic, I was trying to kill off anything that was in it to reuse, it's a really good set up that took me days to do. The new tank was fine, three weeks later I got the little flies again. Kinda like fruit flies almost, that small. Also, I had my scorpions in the old tank for over a year before I got the fly problem in it. :S