Quote Originally Posted by Mr_Cool
I thought the denitrifying bacteria was anaerobic, though. (Well, not really ANaerobic. But, I know it doesn't get much.) I thought that the "risk" of a deep sand bed was that, if the lower levels were disturbed, the tank could crash pretty hard. No?!?
They are anaerobic, but oxygen does not kill them. Otherwise any sand digging critter on the ocean floor would kill them. How did they get there in the first place? Were they teleported into the sand bed from an anaerobic environment in a laboratory somewhere? Actually though, they just shut down their metabolism and take up knitting until the proper anaerobic (or anoxic - low oxygen) environment is restored. Some of these guys are actually facultative, which means they can operate in low oxygen, or high oxygen environments. They have 2 sets of metabolitic pathways, but can only do the denitrification process in low oxygen conditions.

Steve's suggestiion about replacing the sand bed is a good one. A sand bed will not run indefinitely. Eventually there is enough buildup of silt and nutrients that it will crash. When I do my upgrade, I plan on saving some of the sand as a starter culture, and as a lower cost alternative to replacing it, I will kill the sand bed with fresh water and wash the heck out of it. I have done this in the past with pretty good results. A lot of phosphate gets trapped as calcium phosphate on the surface of the aragonite. I may think about doing some sort of mildly acidic wash of the sand to see if I can remove that. I need to see what would be available to do this with. Acid won't work because it would be quickly neutralized by the aragonite and I would just end up dissolving the sand. An acidic buffer might do the trick. I need to see what I can come up with to make an acid buffer. Hmmmm.... Where did I put that freshman chemistry book? Maybe a CO2 sparge.