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Plenums (Re: Plantex CSM Secondary Plant Nutrient Mix Query)

On 2/10/97 12:10 PM, Adam R.Novitt novitt at javanet_com wrote:

>I was also wondering if anyone has experimented with plenums, a la live =
>sand N.N.R. setups in salt, to achieve the substrate water movement that =
>Horst & Horst suggest in The Uber Aquarium.

Around December 1005, I reset up my 3 fr aquarium using a plenum with the 
intention of gaining the nitrate reduction reported in Jaubert-style 
marine setups. Over a period of around 6 months my nitrate levels slowly 
rose to around the 25 ppm mark so my feeling is that they don't work for 
nitrate reduction in fresh water. In the marine systems you place a 
screen above the bottom inch or so of substrate to prevent it getting 
disturbed. I didn't do that and I think the reason the nitrate reduction 
did not occur is likely to be that root growth prevented my substrate 
from becoming anoxic as it becomes in a marine system (the malaysian 
trumpet snails probably don't help either). I also suspect that I have 
plant roots penetrating the plenum and affecting it in ways which would 
not occur in a marine system. Even if you used a screen, I think fine 
hair roots would penetrate the screen over time and you are unlikely to 
get the anoxic zone required for denitrification. I eventuallg got the 
nitrate reduction by removing the biological filter material and turning 
the tank into a "filterless" tank.

What I have noticed is that the temperature of the substrate (shown by a 
liquid crystal thermometer on the glass below gravel level) stays very 
close to the water temperature as displayed on an alcohol type 
thermometer. I have not tried to test the two thermometers against each 
other and I don't know how much I can rely on this. My feeling is that 
diffusion into the plenum increases water flow through the substrate 
above what you would expect in a normal tank without substrate heating 
and that this helps reduce the temperature differential you might 
otherwise expect. If so, you would expect some additional nutrient 
transfer to take place also. On the other hand, I could be fooling myself 
here just to maintain the belief that I'm getting some positive effect 
from the plenum.

I also have no idea whether having a less anoxic/anaerobic substrate is a 
plus or minus for plant growth. There are regular posts on this list 
about the anoxic/anaerobic nature of natural substrates and if the plenum 
is effective in promoting diffusion this will definitely be interfered 
with. I get good root growth on stem plants and echinodorus so, if it is 
detrimental, it isn't excessively so.

I don't know what others might say but I think that for your purposes, 
the $64 question is "Would I set up a plenum again if I was doing a new 
tank or redoing the old one?". I think the answer is "No". What I would 
try is a more complex substrate - soil or something else other than fine 
gravel and laterite balls - and I don't think that would work well with a 
plenum. Fine particles would be likely to sift through the plate 
material. That's probably happened to some degree with my laterite balls 
as they dissolve over time, and also with mulm. I have no way of looking 
at whats actually sitting under the plate within the plenum.

It's been an interesting experiment and a good talking point, but I can't 
point to any results that would make me start strongly advocating the 
technique. On the other hand, I don't think there are any downsides apart 
from limitations on content and fineness of substrate material, and there 
is some possibility that that could be a significant downside.

David Aiken