changing sump, questions

[hummi22689]

Current sump is a 29 gallon that has a broken baffle, cyno growth, and probably a contributor to my nitrates.  I have decided to make a new sump.  It is a 30 gallon (I think).  It is 6" longer, about the same width, and about an inch shorter.  I plan to have a DSB in the middle section (approx 12" wide) with LR rubble.  First, will this cycle through like a new tank?  I am assuming it will, but want to make sure.  How deep should the sand be for a DSB in there? The LS you buy from a big box pet store, is it really LS, meaning does it contain all the little creatures?  Trying to figure out how a bag of sand could conatin anything still living.  Is it better to order from somewhere that tells you it contains "creatures"? 

[Ashlar]

Well, if the LR is from another tank, no, you're not likely to have a cycle. For the sand, if it's still wet, it'll likely be fine. If you want a DSB, I wouldn't go less than 6"- that gives you a decent 'anoxic' zone for denitrification. 8" would be better, but if you can stick to at least 6, you should be good.

Even dry 'live sand' can contain bacterial cysts that will give you a boost over new, never wet sand.

[SapperChris]

Why 6 or 8 inches? I understand the " 'anoxic' zone for denitrification " idea but why not just 4" or whatever? Just asking.

[Ashlar]

6" is the minimum depth normally required to get a zone without oxygen ('anoxic'), with your average sized grain sand and typical flow. At 4" or so, you'll get a 'hypoxic' (low oxygen) zone, and *some* denitrification can happen there, but not much.

Think of it like a layered cake. From 0-4", you'll have a lot of pods, worms, and aerobic bacteria (oxygen loving). From 4-6" you'll have mostly aerobic bacteria that are very good at converting ammonia and nitrite to nitrate, with some anaerobic bacteria at the very bottom. From 6" on down, you'll have anaerobic bacteria that work without oxygen, converting nitrate to nitrogen gas.

[Reefpete]

There are those experts who only recommend DSB's of the 6-8" range for larger aquariums. Personal experience with my smaller aquarium/sump, 4-5". Just sayin' 

[Ashlar]

Ehn, you can't "over"-denitrify. At worst, bacteria populations scale back to meet the available food sources.

You can use a smaller, tighter packed sand and get denitrification at a shallower depth, or slow the flow of water through the fuge to get a similar result. (Of course with lower flow, you get more detritus build-up, too.)