Bob replies...

Dear Arthur,

Thanks for your letter and you are so right about "satisfying some of the folks sometimes, but not all the folks all the time" when it comes to filtration methods. There has been, and still exist today various 'camps' that are set in concrete when it comes to their views. Unfortunately, they will not open their minds and at least admit there are other methods that have admirable attributes that if set up properly and correctly maintained, will provide excellent results. This kind of behavior certainly does not benefit the average aquarist, as "he said and/or they said" type of confusion tears down the fabric of our hobby. And there are some that publicly badmouth fellow aquarists because they have differing stances on various subjects. This 'closed mind' attitude simply causes further entrenchment in some individuals/groups, preventing accord of any kind on various subjects. And this type demeanor is not in the best interest of our hobby!

And even though I have promoted the plenum method since experimenting with it in early 1992, I have used other filtration methods over the last 50 years, such as the Berlin method, with and without deep beds and even bare-bottom tanks, along with various type refugia, and have done so very successfully. So the experience is there to draw upon, and where plenum systems are concerned, they are still sadly misunderstood or misquoted. However, they have been a great learning tool, as you will see with some of the information and facts noted below.

In my opinion, one of the biggest problems within these groups/camps is their terminology and the applied definition of certain words. As you mention in your letter, your thoughts lean towards a very deep bed because, as you say, there will be a greater 'anoxic' area. And sand sifters would greatly benefit that bed. However, that's not totally correct!

When speaking or writing about the terms anaerobic and anoxic, I have made it a point to exactly state what I'm referring to, i.e., anoxic is a zone containing .5 to 2.0 mg/l oxygen where its bacteria, facultative anaerobic heterotrophs, will reduce nitrate back to gaseous elemental forms (mostly nitrogen), which is technically called "dissimilatory denitrification." In the anaerobic area, an area/zone containing less oxygen than the anoxic area, obligate anaerobic heterotrophs produce the ammonification process, or what is technically called assimilatory denitrification. This results in a nitrogen product (ammonium) being returned to the surrounding area. So you see, there are two different zones, below approximately the first half-inch (1 cm) of substrate. Therefore, the majority of the bed in the DSB is truly anaerobic, as the oxic and anoxic zones basically take up the first inch of depth. Accordingly, in the DSB, most of its depth is truly anaerobic and a producer of ammonium, a primary algae nutrient! Great to reduce some of the systems nitrate compound, but worse when it comes to promoting algae because ammonium, another nitrogen compound, is 'the' primary algae nutrient! (Better than nitrate!) Of course, depth of the oxic zone (where nitrification takes place) and the anoxic zone (where reduction of nitrate to nitrogen gas exists) depends upon the sand grain size. However, the smaller its size, the less depth of the oxic and anoxic zones and the greater the anaerobic zone!

Now that we are clear on what and where certain forms of "filtration" occur and why, the next thing to examine is whether or not sand-shifting creatures are of value. The answer is generally yes, except for the larger sand-shifting organisms such as big worms, crustaceans, fish, and starfish that can feed upon the more valuable micro-size crustaceans that ply the substrate bringing nutrients to the various classes of existing bacteria. Unfortunately some of these larger sand-shifters not only diminish (eat) the valuable smaller microorganisms (including the good bacteria), they also can bring too much oxygen into the lower bed areas. And this creats more oxic areas leading to a potentially higher nitrate level in the aquarium, or possibly liberating some ammonium from lower levels.

And I should add all sandbeds eventually have tiny burrowing organisms that compliment the filtration capacities of the system. They usually arrive from live rock and/or coral additions, and find their way into the sandbed. And if we could control their size, activities, and what direction that would take, we would all have amazingly effective microbial colonies in our sandbeds resulting in "balanced" natural aquarium environment. Unfortunately, we aquarists don't have that power and must rely on the known facts of what various filtration methods provide, then match it/them to the goals of the system. And of course, limit the size and type of sand sifting individuals along with sensible maintenance, as you mention, if the sandbed is to remain in a microbially balanced, efficient state of health.

And I must add that Jaubert plenum sandbeds have their share of naturally occurring burrowing infauna/crustaceans. They, unless huge in size as noted above, do not liberate any of the temporary accumulating nutrients found in the plenum area (water space under the grid) from diffusing upwards in the sandbed. In fact, only a major intrusion into the bed all the way down to the grid could possible affect bulk water quality. Even then, I doubt it very much as I have yet to hear of such a happening after keeping plenum systems for 14 years and communicating with hundreds of plenum keepers, including Dr. Jaubert.

Further more, your thought that a Berlin system has limited denitrification and a DSB would help that situation is in error. In fact, is just the opposite of that, as most Berlin systems that I have seen have an over abundance of sand and live rock. And as mentioned above, if the sandbed is too deep, then there is more undesirable assimilatory denitrification occurring, and the same is true inside that of live rock. Probably even worse than that in a deep bed as there is far less penetration of oxygen into live rock! And even if 'nitrate' is a reduced aspect in these type systems, the very real possibility of excessive ammonium production and the inability to test for it along with it being a major algae nutrient, should raise a red flag. Could very well be aquarists with Berlin systems having alga problems have not fully understood the definitions of anoxic and anaerobic and what occurs where and why!

With that said, I think a DSB system is out of the question, and either a Berlin system with a bare bottom or very shallow bed, or an interconnected plenum system should be the possibilities.

As for a refugium, I think it's the area to add the plenum method or at least another shallow sandbed possibly with some macroalgae and at least lit opposite that of the main system. As for strictly a macro refugium (no plenum), they can be any size as there will be some overall system benefits in helping increase alkalinity, pH, and decreasing CO2 levels. Yet any thinking that such an interconnected macro refugium will eliminate the need for a protein skimmer, is not correct. That was an ill advised approach when Alga Turf scrubbers first came out, and after much discussion and further study, the majority of informed aquarists have realized that it is more beneficial to have a quality skimmer than not have one in almost all applications!

Furthermore, as for operating any aquarium or refugium without a skimmer, I find that thinking counter to maintaining an overall balanced microbial system. In fact, almost all aquariums should be equipped with "excellent" protein skimming. The reason for this is that its always possible to reduce skimming efficiency if you have an oversized unit, however not possible to increase its efficiency if undersized. And all systems have bumps in the road where increased skimmer efficiency may just be a lifesaver. And if fulltime skimming is not desirable, I recommend at least using a skimmer during the night when additional oxygen is needed in the aquarium. And if "detrimental effects" of skimming, as you call it, is thought to be the removal of trace elements, there's many excellent brand trace element solutions on the market and any one of them used sensibly would correct this minor problem area.

As for flow rates between refugium and main show tank, that depends upon the purpose of the refugium. If it's for holding a plenum system, anything between half the volume of the refugium and the full capacity of the refugium once per hour is sufficient. And its size, i.e., the size of the plenum tank, should be about half the size of the show tank. However, that really is dependable upon the bioload in the main system. If heavy, than half the size of the main tank is recommended. If bioload is moderate, than a smaller interconnected plenum system will suffice. As for a refugium solely for macroalgae, (no plenum) the flow should be swift enough to stir the macroalgae, yet not cause it to be uprooted so to speak and pushed around the tank or get sucked up into the return flow to the main system. Water motion in the main aquarium can always be enhanced as needed with wave making devices, and its level of flow would depend upon what species are being maintained.

Hope this helps,

Bob Goemans