I've not had good luck with them.  Well if you call 3" of fine sand a DSB.  I always fought nitrates (20 - 100 ppm from water change to water change).  My cichlid tanks looked a lot like a reef tank, lots of rockwork and about the same amount of sand (although I've only used silica).  Doesn't a DSB in a reef aquarium require worms and other bugs to pump the water through it?  I was under the impression that in a freshwater system the motive force in the mud was insect larva.  Maybe it is diffusion???

What do you know/think about the idea that the denitrification in our tanks is occuring in/under biofilms?

jfinch wrote: That interesting.  When I get time, I'm going to tear down my son's freshwater tank and put a plenum in it.

Jon,

Don't go to the dark side.

Deep sand beds work great in freshwater tanks also.

That interesting.  When I get time, I'm going to tear down my son's freshwater tank and put a plenum in it.

I have seen them implemented, but did not personally test. However, the particular display areas looked really spectacular. BTW, these tanks were Tanganican and Malawi cichlid systems. This was also about 3 years ago when I was doing maintanance while I owned and operated my own LFS.

     Jason

Reptoreef and Jon are both successful aquarists, so there must be some value in having and not having a plenum!

I can appreciate the benefits of a plenum in a sump/refugium so I believe that to be a valid idea.  I will probably try it in my new sump/refugium.

Jon brought up the wasted space issue and said it is all about diffusion.  Jon also said that plenums do not work in fresh water aquariums.  Good points too.

The plenum should provide greater exposure to the sand and its' accompanying anerobic bacteria.  Then it would follow that a plenum would be beneficial. 

Maybe, as Jon points out, the answer lies in the middle? 

What would happen if we put more concentrated, oxygenated water through the plenum?

At what depth does the sand in a DSB become less helpful?  2", 6", somewhere in between or can we not achieve that point due to the size of our mini-reef?

Thanks for your help and opinions so far!

The more I think/read about it the more I think a plenum is a waste of space (no offense intended to anyone using one).  The idea behind the plenum is to allow a space for low nutrient water (actual plenum area) which acts as a "wick" to draw more nutrients into the sand bed and it's anerobic bacteria.  The critters in the sand bed are not important... it's all about diffusion.

The theory behind deep sand beds (with or without plenums) is that the bugs in the sand move the water through the bed to the anerobic zones.  Diffusion plays no role.

The truth might be somewhere inbetween.  Maybe.  But if it was all an issue of diffusion then a plenum should work in a freshwater tank too.  And as far as I know they don't.  I think the reason plenums work is because of the bugs and if that's the case then it's wasting space that could be sand.  Just my current thoughts...

Or have I missed something about the plenum?