By Bob Goemans
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Fishy Bizness 

Bob Goemans corresponds with Peter R Winship

Peter R Winship writes...

Dear Bob,

I am currently considering setting up a freshwater plenum system and have recently read with interest the second edition of your book "Living Sand Secrets". In the discussion about preventing light from entering the plenum, you suggest shielding it with sand on either side. Since this sand would of neccessity be approximately 5 inches deep, I was wondering whether there would be any problems associated with the development of anaerobic zones and the accompanying bacterial flora in this region.

Secondly, for aethestic and aquascaping regions in my freshwater aquarium I would prefer to have a thinner layer of substrate at the front of the aquarium. I realise that this area of substrate would not therefore function as effectively from a biological filtration point of view. However, my major concern is whether the thin layer of substrate over this part of the plenum would have more widespread ramifications for the functioning of the plenum as a whole, and therefore for the functioning of the whole sand bed irrespective of its depth.

Any information you could give me on these two aspects of the plenum system would be gratefully appreciated.

Many thanks.


Peter R Winship

Bob replies...

Hi Peter,

Depth of sand relates to the oxygen gradient in the sand. We recommend a 4 inch bed of 2 - 4 mm sand because we have found that size particle when used at that depth to produce the gradient needed to maintain plenum dissolved oxygen at or slightly above that immediately above the grid. In fact, that is the purpose of the plenum, i.e., to retain some oxygen and keep most of the above sandbed in an anoxic state rather than an anaerobic one. If the sandbed were to be more shallow than recommended, the only result would be less anoxic volume, therefore slightly less efficiency for overall aquarium filtration. However, too shallow, less than a couple of inches may render the plenum area aerobic, and if so, there would be no forms of denitrification. Only mineralization and nitrification would exist and nitrate levels would slowly climb.

As for keeping the plenum area in the dark, even though light exists wherever life exists, even in areas 35,000 deep, the visible spectrum at areas where nutrients exist generates algae. It's the reason why algae in one form or another develop around the sandbed edges at the sides of the aquarium. However, those areas can easily be cleaned with a dull knife blade, however the same is not true for inside the plenum area. Also, denitrification microbes are thought to be more productive in darker areas. As for anaerobic areas at the side panels where the sandbed depth is deeper, that does not occur because the plenum grid sides are close enough to prevent it. You may also want to read Protein Skimming and Activated Carbon Secrets; and, Marine Algae Control Secrets. You'll then be totally up to date in these areas.

Hope this helps.



Plenum; Freshwater

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