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Save me... I'm thinking of doing a plenum...

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PDoug View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PDoug Quote  Post ReplyReply Direct Link To This Post Posted: April 02 2008 at 11:24pm
I have not seen such chaos and carnage since the great chipmunk fire of 1979. Interesting debate from what I see Old School Methods vs New If both grow a healthy coral...I was always curious what the arguments were. Now I have to research thanks guysLOL
(307)679-7898 Phillip Douglass 75 Gallon Reef Tank
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BobC63 Quote  Post ReplyReply Direct Link To This Post Posted: April 02 2008 at 11:25pm
Originally posted by Will Spencer Will Spencer wrote:

Finch, he told you where to get it.  Run, don't walk, to your LFS.  Or are you just too cheap?Wink  (You know I'm just kidding, but if you come to the meeting tomorrow you can try to kick my butt anyway if you like.LOL)
 
 
Carlso63.  I don't agree that anoxic is bad either.  According to Wikipedia...

"Anaerobic is a technical word which literally means without air (where "air" is generally used to mean oxygen), as opposed to aerobic.

In wastewater treatment the absence of oxygen is indicated as anoxic."

This says nothing about anoxic always being a bad thing.  I guarantee I have zones in my tanks that contain HS2.  I've never seen any problems from them.

 
So your cool with the possibility that there are places in your sandbed that have water conditions (at least O2 content, anyway) in common with sewage ?
 
If Sukie was reading this she would reply "that's just totally gross, dude!!!" LOL
 
And - since every living thing we put into our tanks, or want to grow in there, from the biggest fish  and corals all the way down to pods and even bacteria - since they all require oxygen to survive and thrive...
 
And - since (conversely) the "bad stuff" (i.e. hydrogen sulfide producing bacterium) need anoxic conditions to survive and thrive...
 
Your still "cool" with anoxic ??? Shocked
 
 
 
 
- My Current Tank: 65g Starfire (sitting empty for 2+ years) -

* Marine & Reef tanks since 1977 *
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jfinch Quote  Post ReplyReply Direct Link To This Post Posted: April 02 2008 at 11:38pm
I don't see why having an open space under the sand bed would lead to better diffusion.  You got any ideas why it would?

Edited by jfinch - April 02 2008 at 11:38pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BobC63 Quote  Post ReplyReply Direct Link To This Post Posted: April 02 2008 at 11:39pm
Diffusion of what?
- My Current Tank: 65g Starfire (sitting empty for 2+ years) -

* Marine & Reef tanks since 1977 *
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Will Spencer Quote  Post ReplyReply Direct Link To This Post Posted: April 02 2008 at 11:42pm
Every drop of water in any of our tanks has conditions in common with sewage.  Do you know what fish do in there?  It is totally gross, dude!LOL
 
Treating that sewage is what I want a DSB to do.  I'm just not conviced it is a bad thing.  I stirred a bunch of Hydrogen sulfide up out of my office tank when I restarted a dead pump on a sand filter and when I smelled it I freaked out.  I spent the rest of the night trying to do a big water change.  Afterward I wondered why I freaked so bad.  I don't think there was really any major problem with it.  (Except the boss being ticked when he came in to the smell in the office the next morning.Wink)
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Will Spencer Quote  Post ReplyReply Direct Link To This Post Posted: April 02 2008 at 11:43pm
PS.  Aren't you supposed to be working? WinkLOL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BobC63 Quote  Post ReplyReply Direct Link To This Post Posted: April 02 2008 at 11:45pm
Shhhhhhhhhhh Wink
 
 
- My Current Tank: 65g Starfire (sitting empty for 2+ years) -

* Marine & Reef tanks since 1977 *
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jfinch Quote  Post ReplyReply Direct Link To This Post Posted: April 02 2008 at 11:56pm
Originally posted by carlso63 carlso63 wrote:

Diffusion of what?


Isn't that what we're talking about?  Diffusion is how water, nitrates, phosphates, oxygen, etc move from the bulk tank water to the bottom of the DSB or plenum.

So, why would having a plenum increase diffusion?

Another thought experiment.  Why aren't plenums all the rage in freshwater tanks?  When I was keeping cichlids I would have killed to have a hands off system that would keep my nitrates below 20 ppm.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BobC63 Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 12:09am

Oh, I (kinda) answered you a few posts back...

It's all about the flow and the water circulation through the bed that a plenum facilitates. Having the void space means you have a water space both above and below your sandbed. This allows nutrient containing water to actually go through the sandbed and past it - to the "other side", so to speak... with a DSB there is no "other side" to migrate to. Just sand, sand, glass = dead end.
 
And IMO the two differing bodies of what have (minutely) differing properties on things like temperature which help facilitate that migration / diffusion of water from one "zone" to the other and back... kind of like the way warmer air rises and colder sinks... well I believe on some scale there is that kind of "circulation" between the zones in a plenumed tank. On a tiny scale, mind you, but I do believe it exists.
 
And better water movement through the sandbed takes all those nutrients with it to the low O2 plenum area, where the denitfiication process is optimized...
 
Lastly, having that microslow "flow" through the bed prevents the compaction of the bed that can occur in a DSB - again, due to the different "flow" of water through the sand instead of that "dead end" effect in a DSB.
 
I don't know if Jaubert & Co agree with my beliefs on why the flow is the way it is; it's just my own hypothesis LOL
 
And I don't know why a plenum wouldn't work in freshwater... maybe there is a reason I am not familiar with... then again, does a DSB - type system work in freshwater? (I don't know
 that, either Embarrassed)
 
 
 
- My Current Tank: 65g Starfire (sitting empty for 2+ years) -

* Marine & Reef tanks since 1977 *
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jfinch Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 12:21am
Ok, so it's the open space under the sand that allows for better diffusion.  How big (deep) does this open space need to be to work?  1"?  1/2"?  1/4"?, 1/8"?, 1/16"? 

As far as diffusion is concerned, I can't see why a 1/2" open space under the bed is any different then just having sand (1/16" open space) at the bottom.

And I don't know why a plenum wouldn't work in freshwater... maybe there is a reason I am not familiar with... then again, does a DSB - type system work in freshwater? (I don't know
 that, either Embarrassed)

Neither work because freshwater substrates don't have the sand stirring critters that ALL saltwater tanks with live rock and coral have.  And in my opinion, all those little critters are THE reason that both DSB and plenum systems work in saltwater tanks.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BobC63 Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 12:31am
Generally 1/2" is the smallest space that will work. And that's on a small tank / small water volume (like 20 -30g)
 
55g - 120g I have found 3/4" is the minimum
 
120g - 240g I would go 1" to 1.5"
 
I know large public aquariums do 4" up to 6" on their plenums
 
BTW, if you were to go with less space it's not like 1/2" won't work at all in, say, a 125g tank; you just don't always get complete dentirification under all circumstances - and eventually, you would find some slight compaction when using a plenum that's too "shallow" for the volume
 
To me, the whole relationship between plenum depth / water volume /  and the effect depth has on how well it works says there is something to the my idea of a circulation zone existance due to differences in the water over the bed vs. under the plenum - be they physical properties like temp or density, or chemical ones, maybe even electrochemical
 
 
 
- My Current Tank: 65g Starfire (sitting empty for 2+ years) -

* Marine & Reef tanks since 1977 *
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chris.rogers Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 8:47am

So, spacing has to do with ability to process waste product in the system because...?

More space allows for more flow?  More space means what, exactly, in terms of processing waste?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jfinch Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 9:06am

Generally 1/2" is the smallest space that will work. And that's on a small tank / small water volume (like 20 -30g)

 
55g - 120g I have found 3/4" is the minimum
 
120g - 240g I would go 1" to 1.5"
 
How many tanks have you operated at varying plenum depths?  Have you ever kept a tank with a DSB?  I've never read anywhere (admittently, I'm probably not fully read up on plenums) that plenum depth was an important parameter.
 
So how important do you think sand depth is?
Grain size?
How should different substrates be distributed?
Do you use screens?
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chris.rogers Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 9:07am
Jon,
 
Are you concerned with long term ill effects of a DSB?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jfinch Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 9:17am
Are you concerned with long term ill effects of a DSB?
 
Maybe LOL
 
Reading through this thread, I'm sure it looks like I'm huge DSB fan and plenum hater, but that's not the case.  I pretty much feel like Jake does.  A plenumn is like adding a golf ball to your tank.  A sand bed is a sand bed.  But I'm certainly open to being wrong but need to understand why.  I think almost everything surounding sand beds is based on conjecture.  Very little real hard science.  Toonen did a very good job in his research (see Jake's post) and he found no significant difference between the two methods.  I'm very interested in reading the magazine article Carlos referenced though.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chris.rogers Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 9:24am

I'm still going through Toonen's article, but I admit I think the plenum is on to something when it comes to allowing the water to filter through the substrate, whereas with a DSB, the lower you go the less water exchange there is.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jfinch Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 9:42am
Chris, you're an engineer.  Read up on Fick's Law (http://en.wikipedia.org/wiki/Fick%27s_law_of_diffusion) and theorize why an open space under the bed would increase diffusion.  I've thought about it off and on over the years and can't see why, but maybe I've missed something.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chris.rogers Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 9:53am
Reading...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chris.rogers Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 10:46am

I'm not sure if diffusion is the correct way to look at the problem.

All things considered, you've got 4 'regions' in a plenum (this is overly simplified but here goes)
 
1.) Water below substrate
2.) Substrate
3.) Water above substrate (water of system)
4.) Live rock
 
All molecules of any sort (phosphate, nitrate, nitrite, ammonia, oxygen, nitrogen, etc...) are going to find their way into an equilibrium inasmuch as the varying processes produce and consume them.  So if you have a source of waste material that is constantly producing its waste and doing so faster than the consumers can consume it, then we'd have a problem.  This brings to mind, among other things, overfeeding and waste buildup.  Lots of us purchase skimmers to physically remove wastes so that the bacteria and natural filtration methods have an easier time doing their job, right?
 
I think the issue/question/crux with the plenum is whether or not it prevents an inaccessible buildup of waste, the way a DSB might.  If there's restricted water movement through the thick substrate of a DSB, then any and all waste producing material (food or fish waste) can conceivably collect in spots that aren't going to be disturbed unless something (your hand or a stick) pokes at it and moves it all around.  If the movement of water through the substrate prevents this buildup, then that could conceivably aid in preventing problems.
 
I do take issue with the statement that was attributed to Mark, the stirring up of sand.  My corals always close up with the murky water when I stirred my sand bed.  Am I to understand it was because they were eating, and not because they were being choked?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jfinch Quote  Post ReplyReply Direct Link To This Post Posted: April 03 2008 at 11:26am
Ok, so diffusion isn't the dominate driving force in getting ammonia or nitrate into the lower sections of the sand.  Let's say it's due in large part to just bulk water transport (caused by all the powerheads in your tank).  I'm ok with this explaination, but I'm still at a loss why a void under the bed helps???  What things affect this transport method?  Grain size, depth, bulk water velocity at the sandbed/tankwater interface, temperature, salinity, ???.  Seems those variables are the same ones that affect the diffusion constant in Fick's law and should be the same for both systems. 
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