I think the sand bed would have to be pretty deep to support anerobic bb
I killed a tank worth of fish with a 20% water change in a deep-bed sand tank.... sand was 3-4" deep... no other ill effects from the same water in 4 other tanks that got WC's at the same time.
I do big water changes in our fish room... 40-80% every 1-3 weeks... basically as often/much as I need to in order to keep up with Bichir Girl's (over