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At PFK we've been tossing around the idea of a freshwater plenum tank - that is a set-up relying on the "plenum" - a gap under a substrate - to filter the tank.

In the marine world these are generally (though not always) combined with "living" sand - a substrate turned over by the living creatures in it - and usually (but again not always) a protein skimmer which removes a lot of the wastes before they can pollute the tank.

The more we read on these systems the more it becomes clear that they DO work in a marine context - and that there are endless variations on the theme, with greatly varying levels of technology involved.

Judging by recent correspondence from the US, there's heated debate still going on about the finer details. The US marine scene (like the Discus, Koi and marine areas in the UK) is packed with controversy and argument. But would a FRESHWATER plenum work? It's hard to see how the substrate could be made live - and standard protein skimmers don't work in freshwater.

We looked for replacements for both. One US expert suggested that plant growth was supposed to be excellent in such systems, which could cover the protein skimmer end of things a little; while the roots would get in among the substrate and slowly break it up - which could be good or bad. Whether the de-nitrifying effect is lost if the roots get into the plenum water is something we'll have to test.

It's hard to think of effective living freshwater creatures to crawl through the substrate and burrow around it. Those that sound good potentially - like freshwater mussels - don't have good survival rates in aquaria, let alone experimental ones.

So here's what we propose to try. We'll use a well lit 24" x 18" x 24" deep tank - small enough to be manageable - and on a base support of pond-sized siporax - the little tubes turned open end up. We'll put in an undergravel plate(s) enveloped in a fine gravel tidy to fit tightly to the sides. The gap or plenum will be under this plate - it may be necessary to use two cylinders of siporax for each support, one on top of the other.  No growing substrate will be placed on top, just fine inert sandy media as used for planted tanks.

We'll plant this with fast-growing hardy aquatic plants with good root systems. The tank will be well-lit but there will be no other filtration or equipment apart from a heater. We'll stock lightly - probably with small guppies (of which, not surprisingly, we have plenty) and test the water chemistry constantly.

The only problem with this set-up is that an ultra-modern plenum style tank has actually ended up close to the totally-traditional "balanced" style tank as recognised by the older generation in the hobby and covered a few times in PFK. This type of tank relies on the plants to complete the filtration cycle. Whether the addition of a plenum will "improve" it remains to be seen.

© Steve Windsor. May not be reproduced in part, or whole, without permission.

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