Saltcorner
By Bob Goemans
By Bob Goemans
Hi Bob,
I don't know if you keep records as you probably get 100's of emails, but about 6 months ago I emailed you about my new NNR set up. At any rate, I upsized from a 90 to a 250 gallon tank which was going to initially be a fish-only tank, and still is. I set it up so that I would be able to switch to a reef somewhere down the road, with just the addition of better lighting and maybe a calcium reactor.
I read your booklets and set up the NNR system as described. I have a plenum in the aquarium and about 120 pounds of Fiji live rock which is supported on a flow through grid just below the top of the sand. I also converted an old 50 gallon aquarium to a sump with a NNR sandbed. The sump is simply a flow through from the aquarium to a second sump, which houses my protein skimmer and feeds my main return pump.
The system has been up and running for about 6 months and so far things have been great. My nitrates have been below 10 ppm and everything else is exactly where I would like it to be. Unfortunately I have had to dismantle the sump with the NNR sandbed to replace one of the end pieces of glass that developed a small crack (the joys of saving money by using an old aquarium). While the filter was running I noticed that the bottom 3 to 4 inches of sand was a darker color with a black tint to it than the top inch or so.
When I dismantled the filter I noticed a faint but definite sulfur smell when I exposed the bottom 2" of sand. There was no smell from the filter before I exposed the bottom two inches of sand either while it was running or even when it was drained. I separated as much as possible the layers of sand. I have the sand from above the top screen in one container and the sand between the top screen and the plenum in another container. Both containers have the sand covered with salt water. I also took water samples from the water that drained from the plenum area and tested it along with the water from the main aquarium. The water from the plenum was virtually nitrate free while the main aquarium had about 5-10 ppm. The pH might have been slightly less in the plenum but everything else was the same as the main aquarium, i.e., zero nitrite and ammonia. There didn't appear to be any sediment in the plenum area other than the sediment from the sand that I didn't get rid of completely when I initially washed the sand before I placed it in the filter.
My questions are (finally you say)!
1. Is it natural to have the faint sulfur smell in the area of sand which isn't exposed to oxygen and should I be concerned about it. Is it possible that this could develop into something that might cause my tank to crash much like what can happen with undergravel filters.
2. My original plan was to replace the sand in the same relative layers without rinsing it at all. Now I am wondering if it should be washed thoroughly with fresh water before I replace it to eliminate the sulfur smell and start fresh again. I realize that this will destroy all the bacteria that have developed over the last 6 months, but the smell does concern me. The smell is fairly faint but I can notice it especially when I place my nose close to the sand.
3. If this isn't a normal occurrence how do I avoid it happening again? Actually, I would not even have been aware of it had I not had to replace the piece of glass. In a worst case scenario this might have been a disaster in the making and I would really like to avoid that in the future.
I look forward to your reply and want to thank you in advance for any advice you might offer me. I really appreciate your help.
Dave Lineker
Hi Dave,
Hundreds of emails per month is about right! And thanks for an interesting and well written letter about an area of concern many aquarists don't fully understand - the sulfur cycle. I've heard so many different positions on this subject that it's no wonder there is much confusion. Some aquarists think darker colored sand is the beginning of the end, others blame their existing problems on it, others say a particular method is the sole cause, etc. and etc.!
Actually, the process that causes some degree of hydrogen sulfide (H2S) is a 'very normal' happening in any type bed, especially those that exceed a few inches in depth and/or even shallower beds of very fine-grained substrate.
Let me begin by first explaining the sulfur - sulfate process, however, its going to get somewhat technical.
The sulfur process is actually a never-ending pathway or mechanism both in the wild and in the aquarium. It moves from sulfate to sulfide, then back to sulfate with the help of various types of bacteria. Dissimilatory Sulfate Reducing Bacteria (SRB), using excretion and decomposition produce sulfide. The resulting sulfide can be re-oxidize back to sulfate with the help of Colorless Sulfur Bacteria (CRB) and Purple Sulfur Bacteria (PSB). Again, this normally occurs in sandbeds deeper than a couple of inches and especially in fine grain beds. To what degree depends upon the depth of the bed, its physical grain size, and whether there is a plenum or the bed sits directly on the aquarium bottom.
Keep in mind that the activity of aerobic heterotrophic organisms in the upper sand level deplete oxygen, and fermentative organisms in this rich area of organic material provide the growth substances for the SRB. Whether in plenum-equipped aquaria, sandbeds directly on the aquarium bottom, or in the wild, the process is the same - the SRB reduce the sulfate to sulfide. In fact, the SRB move upward in the sand column at night to get more oxygen since the oxygen level in general decreases at night. Should there be some areas near the substrate surface with very little or no water movement, fish deaths could occur due to low dissolved oxygen in the area where they rest at night. This is another good reason to have adequate water movement in closed systems that utilize any type sandbeds.
As for sulfide in micro-areas where there is no oxygen, PSB will oxidize sulfide to sulfate provided there is light. Where there is excess oxygen, almost all sulfides will be oxidized back to harmless sulfate by CSB. Actually CSB has higher affinities for sulfide than PSB.
As I see it, the 'Chicken Little' attitude regarding H2S is on the fringe because the natural ecology of the sandbed is to maintain it and keep it from causing a harmful release unless stirred up mechanically. Actually, H2S has always been given a bad rap because the somewhat blackish coloration of the sediment was/is equated with a drastic, life-ending situation that was soon to occur. And often the filtration method was blamed, not the real cause of the situation. Which could simply be the placing of flat rocks on their flat sides directly on the sandbed surface, therefore blocking the penetration of oxygen. Or blocking the interface of sand and bulk water by allowing the bed material to cake-up and form solidified/concrete-like chunks.
To carry this thinking somewhat further, typically there's a natural need for sulfur bacteria in sandbeds, as they comprise several important microbial groups just under the sediment surface. They occupy much the same vertical profile and space the nitrogen bacteria do and they are linked to metabolism in the carbon cycle. In fact, sulfur bacteria are very much needed in the bio-geochemical pathways that are crucial to energy cycling and the equilibrium mechanism that produces balance in the closed environment.
Actually, sulfur 'metabolism and pathways' are not really a cycle, but more a process which can also tie up (store) excess carbon. When the nutrient flux is on the upswing, but metabolism is slow or at its limit, a natural storage effect occurs. Ideally there should be a portioned relationship with nitrogen, often referred to as the Redfield Ratio (C:N), which is about seven parts carbon to one part nitrogen. When an increased amount of carbon shifts the ratio, sulfur bacteria can temporarily store some of it, thereby buffering the flux. In the wild this has been linked to storage for future use, such as in spring algae blooms.
And, as an interesting side note, sulfur bacteria may be linked to the movement and use of phosphate. Classically, in numerous articles it has always been assumed that phosphorous released from sediments is an abiotic or kinetic process. In other words it happens physically, not biologically by microbial mediation. It now appears that phosphorous (including phosphate) under reduced oxygen conditions links ferric (iron) minerals to reduction thereby producing ferrous ions with a co-release of the phosphorous that is absorbed to the ferric mineral.
When the metabolism of bio-available carbon occurs, it in turn causes an increase in oxygen consumption and a concurrent reduction in the redox potential of the sediment. The iron then present in this environment reduces and in the process, phosphorous is released from coupled iron surfaces. Bear in mind that in reduced oxygen environments the electron acceptors between sulfur and iron are closely coupled, thereby making phosphorous release very possible. But don't get too depressed as some of this is designed for uptake into energy production components such as ATP. However, it is reasonable to assume that if equilibrium is shifted with abundant iron, carbon, sulfur ions and a redox that is shifting too far negative, the result would be a release large enough to override equilibrium and favor phosphorous users like algae. Instead of the gain being energy, phosphorous then goes to primary production of plant tissue. If so, one could assume that the use of iron additives and/or media rich in iron may be less than a good idea when it comes to maintaining an algae free environment.
To follow this line of thinking it might then be somewhat perilous to have a sandbed with a large stagnate area of sulfur-laden sand. A deep sandbed without a plenum is a good example as it has a highly favored location for anaerobic microbial mediation. This would equate to a larger carbon storage area of less productive anaerobes. A situation now more favored for phosphorous release than ATP production. In such a case, when sulfur bacteria decouple carbon from the carbon cycle for storage, iron may also accompany the mechanism to facilitate phosphorous release. A large carbon flux shifting the C:N ratio would facilitate this condition and the resulting reaction. This effect fits many case histories and observations in the aquarium where nitrogen nutrients and redox are at low levels but there are hair algae blooms on the substrate (rock or sand). An even worst example would be an iron rich, swamp-like mud-like sediment! And if so, methane could now enter the picture, which is a very possible situation that unfortunately many don't think about when very fine mud-like sediments are used.
Getting back to your situation, having a nitrate free plenum space in the sump tank indicates your plenum bed was not functioning correctly. It could be that the area under the screening was totally oxygen free (anaerobic), which is the cause of the darkening sand/H2S smell. Normally, a plenum system, because of its oxygen content in its bed and the plenum space itself, differs from that of the wild and that of deep beds directly on the aquarium bottom. However, if the pathways are blocked/clogged, it begins to function no different than deep beds directly on the aquarium bottom. In benthic sediments in the wild the chemical pathways are somewhat stagnated due to the lack of oxygen in deeper zones. However, with a plenum the aquarist has manufactured an area of considerable volume, which is in steady flux thanks to diffusion gradients. Even though plenum mechanics are fairly straightforward that doesn't mean they are always perfect. Sometimes the dividing screen, which isn't always needed, clogs with excess detritus and/or possibly some very fine sand - something below 1.0 mm. When that happens, H2S, because there is less diffusion and/or bioturbation, will depict its presence. Whether that is a growing problem or something the system will be able to naturally deal with in the long run, is an unknown as every system is different, just like people. Yet, most of the time, H2S is not released into the bulk water unless the sand is stirred. However, I would think about leaving the dividing screen out if there would not be any digging animals in the sump. And simply wash the sand with freshwater and reuse it as the bacteria species that will inhabit it in the future will be a different species than that which was living in the darken sand.
I question the need for a 'flow through grid just below the top surface of the sand' in the main system, as that may become a major obstruction to diffusion should that sand area become solidified. Precipitants and/or mats of bacteria can cause sand areas to become concrete-like. If you can't breakup those clumps of sand, the area under that hardened layer will become anaerobic and your nitrate level and possibly unwanted algae growth would increase, as would H2S increase. And, that will not be the fault of a plenum design!
Checkout my new website at www.saltcorner.com and hope this helps,
Bob
