Hello all,
Sorry if I got a bit nasty in some of those exchanges.
My feeling about the sub-sand plenums is that they take up a lot of vertical space. If you have a 24 inch deep aquarium, perhaps a 75 or 90 gallon, the appropriate sized plenum for a primarily invertibrate reef type tank will occupy about 6 inches of depth, assuming 2inches for the plenum and 4 inches of sand. For an aquarium with a substantial fish population, even that size may be inadequate.
I think that what ultimately matters are the outcomes; what actually happens inside the aquarium over time. Any good system that is utilized should do several things; some of the most important are that:
it should immediately eliminate ammonia and nitrates; it should reduce nitrates to a reasonably low level in a fish only tank, and be able to eliminate nitrates completely in a reef tank, even one with a small fish population; it should keep other parameters of aquarium chemistry, like alkalinity, ph, essential trace elements, oxygen levels, organics, etc., within acceptable levels assuming regular, though not excessively burdensome, water changes.
The filtration and water quality enhancement processes sub-sand plenums offer can be replicated, in my experience, through the use of thick, porous live rock, in both the aquarium itself, and in a capacious sump. Nitrate reduction will take place very efficiently within the rock core, as well as in other areas. A very fine "live" sand bed no more than an inch thick in the tank and/or the sump is also very effective, especially if some of the sand is replaced once or twice a year. If you are fortunate to have accesss to naturally occuring coral sand, it will come with a ready made population of stirrers and movers. Small crabs, hermits, etc., are very efficient. Various sea worms are probably the most useful, and provide natural food sources, along with the tiny copepods and other minute life that live in the rock. Seeding works, but is much slower, and provides a much less diverse population of tiny critters. After a year or two, the aquarium will develop its own ecological balances, especially if it has few or no fish present. A couple of gobies, Grammas, and the like are no problem.
I have nothing against the subsand plenums developed in semi-open systems in Monaco, except for the vertical space consumed. I also think a reef tank with a good amount of live rock will eventually stabilize into a system that does everything the Joubert type systems do, if bio loads are kept low, the right sort of thick rock is used, and there is good strong water movement in all parts of the aquarium. Water changes are essential in any system, and should provide sufficient trace elements in a system with lots of live rock. Additives for alkalinity, redox, etc., should be kept to an absolute minimum. They tend to create reciprocal problems that feed on each other and create unnecessary endless cycles of measurement, addition of chemicals, more measurements, more chemicals, more worry, more measurements, more additive adjustments, ad infinitum.
Change your water. Let natural systems develop. Keep the water moving. Keep bio loads very light. It's a simple formula that has worked for me for a long time. Over the years I discarded all sorts of reactors, generators, and other stuff. I have found that simplicity and naturally occuring natural processes work beautifully. In addition, various catastrophies and crashes are less likely. Natural systems like the kind I described can also handle power blackouts much better than more complex arrangements.
