By Bob Goemans
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Bob Goemans corresponds with Brendan McKelvey (England)

Brendan McKelvey (England) writes...

Dear Bob,

For about 28 years now I've been an avid fish keeper. But for the last two years I have kept a small reef tank, parameters are nitrite 0, nitrate 10 ppm, ammonia 0. My question is, I need a bigger system from my 180 litre to a 300 litre, I have built the sump with a beast skimmer with a return pump of 2800 lph. But I wanted to put in a plenum, but my local shop said a D.S.B would be better with macro algae, but what depth do I have substrate?

Whilst I have been an avid reader of your articles for many years now, this is the first time I have actually written to you. Hopefully, this note will make my question that bit clearer.

I have found myself in a bit of a predicament. Do I go for the plenum or the deep sand bed? The sump is 36 inches long, 15 inches wide and 18 inches high. Water enters into a built in 6x6 inch glass wet and dry tower, which is 22 inches high, giving 2 inches at the bottom for water to escape.

At the side of this is a large pump 2800 lph, to power the Beast skimmer. Next to the pump venturi, water then goes over a 6 inch high 15 inch wide water deflector. But then I have 29 inches in length by 15 inches in width to use as a sand filter. Media will be a mixture of 2/4 mil aragonite, approx. 4 inch with whatever live sand I can obtain on top.

Please help me. I have heard bad reports on both, but I do want a naturally filtered system.

I have given you the sump size; 36x18x15 inches, but the main tank is 36l x 24h x 24w, and again, it will be filtered as naturally as possible, using about 1/1.5lb of good live rock for bio-filtration per gallon of water.

Total turnover of water will be 33.3 times per hour using the return pump power heads x 2, ex-filter, skimmer-pump and spray bar pump. Total is 10,100 litres per hour (approx.). Lighting is a mix of T8 tubes, 2 blue and 2 10,000K; also power compacts 'twin 55 watt' 10,000k, total 230 watts.

Does all this sound like a good set-up?

My present set up has many soft corals, xenia, cactus anemone. They are all constantly multiplying.

Is it true that if a D.S.B is not deep enough it will produce nitrate rather than produce nitrogen bubbles?

Also, one last thing. Do plenums produce hydrogen sulphide? I have asked two experts and got conflicting answers. My mind goes with the plenum, but please give me the s.p. on both.

Thanks for taking the time to read this. I very much look forward to your answer.

Yours truly Brendan McKelvey, England

PS - Would if be too much to use two skimmers in my sump refugia?

Bob replies...

Dear Brendan,

Thanks for your mail and as for a deep sandbed being better for macroalgae, that would be correct. That's because deep beds have a greater production of nutrients, such as nitrate and ammonium. I know many think that nitrate is totally consumed in the depths of deep beds, however, little do they realize there is a greater production of ammonium, the most important algae nutrient, and that a portion of that is reconverted to nitrate.

Keep in mind the bacteria in the upper most area of the sandbed feed upon ammonia in the bulk water that results from animal waste. These aerobic autotrophs reduce it to less toxic substances such as nitrite, then to nitrate. It's called the nitrification cycle. Then deeper down in the bed the resulting nitrate is acted upon in an area that contains little or no oxygen where it is reduced to either nitrogen gas or ammonium. To do so relates directly to the depth and physical size of the sand grain. If the sand falls into what is generally categorized as course sand, i.e., 2 to 5 mm grain size, the aerobic autotrophs live approximately within the upper half inch (1.5 cm) of sand. If the sand grain is finer, there is less depth to their colonization area because they need the oxygen passing down through and around the sand grains. Simply put, the smaller the grains, the less space between them, therefore the less diffusion/penetration of oxygen. In mud-like substrates, their depth of colonization can be paper-thin. This simply means more nutrients can remain unprocessed in the bulk water (and also be produced in the mud). Good for those forms of life that like that, such as algae.

Once below this depth, the area begins to naturally contain less oxygen (it's mostly used above by the aerobic autotrophs) and a different class of bacterium inhabit this area. Again, if the sand falls into what is generally categorized as course, there will now be approximately another half inch where the oxygen content is now very low, i.e., approximately 0.5 to 2.0 ppm, creating an area rightly called the anoxic zone. This is where facultative anaerobic heterotrophs reduce the incoming nitrate to nitrogen gas. And if the sand is much finer, the depth of the very valuable anoxic zone would be much shallower, and probably be nonexistent in mud type substrates.

Below this depth, and where oxygen is now less that 0.5 ppm, often called the anaerobic area, obligate anaerobic heterotrophs reduce nitrate to only that of ammonium, not nitrogen gas. This is extremely important since ammonium can be recycled in that area back to nitrate and either or both nitrate and ammonium can diffuse upward in the sandbed, creating an extremely nutrient rich bed and very possibly leaching into the bulk water. And since any depth approximately below the first inch in course sand contain the obligate anaerobic heterotrophs, which accomplish something technically called assimilatory denitrification, nitrate is only reduced to ammonium, no further! And in finer grained environments, most all of their depth house this class bacteria. Therefore, it should now be extremely clear just what class bacteria exist where and why, and what they accomplish, and why deep sandbeds are better for various type algae.

Now that you have the "FACTS" it all depends upon what you want to accomplish. If the goal is a nutrient poor environment, than a shallow bed in the main aquarium with a reasonable amount of live rock (one pound per gallon) and a truly separate interconnected plenum where flow to and from it does not exceed its volume once per hour is the way to go.

In my opinion, the sump tank you now have will not serve as a good place to install a plenum, as this entire tank is now serving as a place for system equipment. An interconnected plenum should be just that, a separate place for the plenum to accomplish natural filtration. Not sharing its space with major equipment causing major water flows/disturbances.

And if the goal is to maintain macroalgae, your present system should accomplish that since its already somewhat nutrient rich. Could be the space you thought could serve as a plenum area would be better used as a lighted macroalgae area with a fairly deep bed.

As for the hydrogen sulfide issue, there is far less in plenum beds than what occurs in deeper beds directly on the aquarium bottom. That's because the plenum bed contains a far greater amount of oxygen! And that is an incontestable 'fact!'

All in all, your system appears to be healthy as is, and don't see the need for the addition of a second skimmer. In other words, if it ain't broke, don't fix it!

Hope this has answered all your questions.

Bob Goemans


Sulphur; Plenum; Hydrogen Sulfide; Deep Sandbed Method

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