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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