Bob replies...

Hi Stanley,

Thanks for a very good letter and insight to what's going on in your area. Let me first address the issue as why a plenum system should be more efficient than different depth beds directly on the aquarium bottom. Let's first think about "balance." When one thinks of "balance" a mental picture of an equal amount of something on either side of a dividing line comes to mind. Lately, the words "microbial balance" are being tied to biodiversity. The thought there is the closed system's environment needs to be in sync with what is found in the wild. Actually, aquarium microbial balance has nothing to do equal sums or the vast diversity of organisms found in the wild. To be truly successful with closed systems the microbial foundation of a closed system needs to contain the correct "proportions/volumes" of microbial processes that use "most of" the incoming energy/nutrients so major accumulation does not happen. Also, it must be clear how the words 'anoxic' and 'anaerobic' are being used in one's writings, and that is why they are defined in my booklets.

Having a closed system where microbial areas produce more nitrogen products, e.g., nitrate/ammonium, actually helps skew microbial 'balance' in favor of accumulation. At sometime in the future that's going to rear its ugly head and no-doubt become a maintenance headache, probably in the form of unwanted algae as algae are nothing more than a green form of those excesses. This is the reason why I prefer sandbeds that do not contain too much anaerobic volume, because that's where a class of anaerobic microbes exist that only reduce nitrate to ammonium, no further. However, the plenum sandbed retains a small amount of dissolved oxygen, therefore it continues to stay in an anoxic, not anaerobic state. The same would be true of very shallow beds directly on the aquarium bottom.

Even though many say I'm a proponent of a certain kind of filtration, I still believe an aquarist can be successful with any type filtration system, e.g., undergravel filter, deep sandbeds, trickle filters, fluidbed filters, plenum systems, etc. It all boils down to understanding the limitations associated with different processes/methods/equipment in relation to bioload. However, I'm a person that wants to better understand the more 'efficient' natural pathways and utilize them in a closed system to their fullest so as to establish a meaningful natural balance. And I have found with the help of Sam Gamble, a marine biologist, that understanding plenum mechanics before turning to other methods has opened the door to having a much better, balanced system.

Besides trying to limit anaerobic areas, another fact to consider is there must not only be proper oxygen gradients in the sandbed, they must be accompanied by the proper sequence of electrical charges for efficient use of energy. Deep sandbeds used directly on the aquarium bottom do not contribute to this situation. The deeper the sand, the more negative it becomes because of its lack of dissolved oxygen and the more the positive charged nutrients are attracted to the lower more negative depths. But the same end result is not true for the plenum system because one of the main purposes of the plenum void/space is to retain a small amount of dissolved oxygen, thereby having a slightly less negative redox than the sand above it. And yes, the sand in the plenum bed also becomes more negative with depth. However, its change in the sequence of the diminishing electrical charge at the void sends elements back up into the bed for proper reduction, recycling elements instead of accumulating them as what can occur in deep beds on the aquarium bottom. Plenum beds simply accumulate fewer nutrients because their microbes are more efficient. You may want to show this email to your storeowners! Fact is fact! Hopefully they will be enlightened.

I'll now answer/comment on your questions;

There are two different methods for reading nitrate. Some nitrate test kits measure the "nitrate ion" (such as the Tetra kit) and others measure "nitrate nitrogen." Nitrate is a compound, or combination of elements. One molecule of nitrate is composed of one nitrogen atom and three oxygen atoms. Since the atomic weight of nitrogen is 14.01 and the atomic weight of one oxygen atom is 16, the weight of one nitrate molecule equals 62.01, or 14.01 + 16 + 16 + 16. Therefore the test kit that reads the nitrate molecule/ion will show a reading 4.4 times higher than a test kit that reads nitrate-nitrogen (such as those from Aquarium Systems). I mention this so you can accurately judge your nitrate level. Anything above 10 to 15 mg/l or ppm should be considered too high. And plenum systems, if constructed correctly, should easily keep it much lower! What is your nitrate reading? (Note, Stanley's reading in his new tank was about 15 ppm and he was using the Tetra kit. Therefore his nitrate nitrogen reading was only 3.4 ppm. And, therefore, the nitrate level in his fish-only tank was also far less than he thought.)

If not nitrate, what else could be causing the algae in your tank? Could be all that shell material, which is normally high in phosphate, has helped generate algae spores that came in on your live rock. This is a good reason why calcium carbonate material containing shell material should not be used in calcium reactors! Moreover, ammonium is normally available at the surface of almost any substrate/bulk water interface, whether that be sand or rock as that is where the nitrification cycle is beginning. Therefore algae on a fairly new sandbed surface is often to be expected. Try siphoning out the surface algae and some of the upper most sand grains as often as necessary until the microbial processes become more established. If your sand is slightly larger than 4.0 mm, add an extra inch to the depth of the bed so as to lessen oxygen penetration/diffusion into the bed. Stay after the algae and mechanically remove where necessary. In a few weeks the system should begin to balance itself. And, I'm not sure where the thought came about that two different size sand grains are needed for plenum systems, but that is incorrect. One correct size is all that's needed. (Note, Stanley followed these recommendations and his system became algae free within a few weeks.)

As for water movement, it depends upon what inhabitants are being maintained. Soft corals generally do better with less water movement than those sturdy stony corals found on fringing reefs. My rule of thumb is three times the volume of the tank per hour for soft corals, five to ten times for stony corals, all accomplished in varying surges. However that doesn't mean that volume should flow directly against the coral itself. That's a generalization for the entire tank, and Dr. Jaubert is simply stating flow that directly impacts the animal. Yet, trying to duplicate in a closed system the same flow that impacted the animal in the wild is nearly impossible. But keeping it mind when establishing a system, is useful.

Your question on an auxiliary tank is not clear. Are you saying this would be a plenum system and interconnected to the larger fish-only system? If so, set up the smaller plenum system and connect it immediately. If the sand in question will not fall through the screen material on the plenum grid and its particles are in the range of 1 to 2 mm, a three to four inch bed would be okay. As for the flow from the small interconnected plenum tank, it should be about half its volume per hour. I should note that auxiliary interconnected plenum systems are becoming very popular as they are easier to maintain and the main tank can then have a shallow bed. And don't draw water from the plenum void, as that would probably establish a fully aerobic bed and void where only nitrification would occur. Even if disconnected a month down the line, the establishment of natural pathways has been delayed and overall system efficiency is now delayed or severely impacted. Could be the impact from that would be far too many nitrogen compounds and skew system balance from the get-go. To shorten the cycle time any of the products helping to quickly establish the bacteria is useful. Hagen's Cycle is a good choice.

I hope this has been helpful, and enjoy,

Bob