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Re: Plenum method

On Fri, 28 Nov 1997, Cliff Mason <lcmason at ici_net> asked:

>I will be setting up a 90g tank soon and I was wondering if anyone has
>applied the plenum filtration method used in mini-reef tanks  to a
>freshwater system


I did in my 3 foot tank, but I did not do it exactly the way the 
reef-tank people recommend.

What I did was to use a UG filter plate as the method of creating the 
plenum, and throw away the uplift tubes. As a result, the height of the 
plenum is only around half of what the reef tank people recommend.

I also did not use screen material midway in the substrate to create a 
bottom area that would not get disturbed. Plant roots and Malaysian 
Trumpet snails have full access to the whole of my substrate.

I did not get the denitrification I hoped for, and the above differences 
may well be part of the reason for that. I mention another possibility a 
little later.

My recommendations, based on my experience, for anyone interested in 
trying a plenum in a planted tank are as follows:

- make sure the plenum height is 1 to 1.5 inches as recommended in the 
reef tank material,
- place the plenum in a sump where you don't have to worry about having 
roots or burrowing snails disturb it, and allow plants and snails full 
access to the tank substrate.

As an alternative, simply place a mesh-type container (a filter bag or 
something) of Sipporax or other highly porous filter media in the back of 
your tank out of sight and forget it.

Note that I haven't tested the first recommendation re placing the plenum 
in a sump and I  have not heard of any results of someone trying this. 
I'm simply suggesting the simplest way I can think of implementing the 
reef tank recommendations in a planted tank while allowing the tank 
substrate to function normally. I have seen comments in reef tank 
articles/books that you can set up a plenum successfully in a sump.

The second recommendation basically comes from a posting by Jean Olson in 
this Digest some months ago, and she indicated that using that technique 
produced denitrification in her tank, but that it took quite a few months 
to achieve effectiveness.

Note that in a reef tank with a plenum, there is usually no "normal" 
biological filter. That work is done by the total combination of uptake 
by organisms like corals, protein skimming to remove nutrients in their 
original form before nitrification can occur, and by the live rock and 
substrate where water movement is quite slow since it occurs by 
diffusion. The actual level of nitrate formation is quite low already. I 
think another part of the reason why the plenum approach didn't work for 
me may well be that a standard bacterial biological filter can actually 
produce more nitrate than a plenum can handle. My nitrate levels did not 
seem to build up as fast with the plenum as they did previously without 
it, and it may well be that the plenum was working but only slowing 
nitrate buildup since it could not remove all of the nitrate.

An even simpler way of achieving massive reductions in nitrate levels in 
planted tanks is simply to remove biological filter material and rely on 
the plants to uptake ammonia and remove the need for the nitrification 
process. There have been a number of postings and articles on 
"filterless" plant tanks and that is the way I eventually went, although 
the plenum is still in the tank simply because I did not want to break 
the tank down to remove it. My nitrate levels are now virtually 
unmeasurable and the tank looks better than it did when I was running a 
normal filter. Note that best results with this method rely on sufficient 
plants to remove the ammonia produced by the fish so start with lots of 
plants and a low fish load. If you wish later on, you can try increasing 
fish numbers over time as you see what your tank is capable of supporting.

I hope these comments help. I'm happy to correspond further on my 
experience off-list if you wish.

David Aiken