By Bob Goemans
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Two Little Fishies 

Bob Goemans corresponds with Joe Smith (London, United Kingdom)

Joe Smith (London, United Kingdom) writes...

Hi Bob

I’m a plenum to be user, and am wondering about how distant the side of the plenum grid should be from the sides of the aquarium? Also, you have said in past writings that the eggcrate used to make the grid is strong material - agreed - but the 'sides' of this grid are covered with screen material are they not? If the grid is supported by short vertical pieces of PVC tubing in its corners and several other places as needed along its sides, the screening that wraps the entire grid is very weak between these supports and could possibly break. Would you recommend tying tubing all the way around the edge of the eggcrate base to make a rudimentary surrounding wall for the entire plenum grid? From the diagram in one of your booklets it looks like you are using short vertical lengths of tubing as eggcrate support, which would imply flimsy bits of screen wall between those support pieces. I hope its clear as to my concern.

And finally would appreciate your thoughts on the use of external filters, or sump systems when using a plenum, as long as they do not contain too much biological media? But then does this not mean you must limit the amount of live rock as well?

Thanks again for your time - it means a lot to me and I'm sure hundreds of other people around the world!

Many thanks

Joe Smith

London, United Kingdom

Bob replies...

Hi Joe,

Thanks for your letter and good questions when it comes to applying the plenum method. In the early days of plenum usage, the eggcrate was elevated with short vertical pieces of PVC (legs), and even though there was some sideward pressure from the sand surrounding grid sides, there was not enough pressure between its vertical supports to tear the screening. But others voiced your concern in those early days and the picture frame concept came about. It was a better approach, and in fact allowed the sand between its sides and aquarium side panels, which should be about 1 cm, to be safely stirred as needed to remain clean looking. I use a chopstick to clean the side areas of my plenum-equipped aquariums. And of course, plenum location is to be in an interconnected sump/aquarium, as it’s too difficult to maintain properly in the main show tank as noted in my past writings.

As for the frame material, small dimension PVC piping with holes drilled through it every 2.5 cm will suffice to prevent stagnate water inside the piping. Plastic cable ties can be used for attaching it to the eggcrate. Some short pieces of tubing used vertical or horizontally should also be cable tied at some central areas to provide support for the above sandbed and live rock. When wrapping the grid, place the ‘whole’ assembly with its top surface facing downward on a large sheet of screening and wrapped like a present, with its overlapping edges of screening on the bottom of the grid. This way, only one layer of screening is on its top surface, which is the way it should be so as to prevent clogging its tiny openings. And I should add, the picture frame concept has proven popular with all and is the way to go. In fact, custom-made plenum grids, as pictured, are available from

As for live rock, when it comes to the nitrification cycle, the plenum process is far more efficient and useful than what occurs in live rock or in a sandbed directly on the aquarium bottom. Nevertheless, live rock provides a more natural, reef-like looking environment, and can also provide some useful microcrustaceans. Therefore, limiting rock use when using a plenum system is quite feasible (and saves money), but to what extent is up to the hobbyist. As to other forms of biological media, such as bio-balls or porous ceramic media, they are not needed at all, as the plenum method, if correctly established and maintained, will adequately maintain near zero nitrates. So duplicate nitrification schemes are not needed. The only other filtration needs will be for chemical filtration, such as provided by a canister filter or phosphate reactor.

Hope this answers your questions,





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