There is a widely held belief (based on common observation and experience) that after several years of success with growing corals, a reef aquarium may slowly or suddenly no longer support the healthy coral growth it once did. This general observation has been dubbed “old tank syndrome.” Various authors have attempted to unravel the mystery regarding the cause(s) of this decline in reef aquarium vigor. Others have pointed to reef aquariums that “keep on going and going” like the Energizer™ bunny, as proof that old tank syndrome does not really exist.
Does old tank syndrome really exist?
Because so many aquarists have experienced it, old tank syndrome must exist. However, as with other syndromes, the causes are many, so it is not a simple matter to say that this is just one problem since it isn’t. This article will examine the factors that I believe to be involved, and discuss their relevance to what actually happens in reef aquariums. This article does not represent the results of experimental analysis. It is instead a summary of some ideas that have been presented in the aquarium and scientific literature, a discussion of their relevance, and a discussion of some observations and proposals of my own regarding old tank phenomena.
There are some common trends in reef aquariums that we all know about. Nevertheless it is natural to operate our aquariums the way we steer an auto driving down a long straight road. Very subtle changes in direction that are hard to perceive can suddenly result in the car running off the road if you don’t pay attention and the steering wheel is left in one position. This analogy relates to shifts in water parameters that can catch you by surprise if you become complacent and don’t pay attention to them at least once in a while. Paletta (2006) discussed the topic of old tank syndrome and provides excellent coverage of all the various types of “benign neglect” that can send an aquarium off course and result in a change in the way its inhabitants survive or thrive. I will revisit some of these ideas here, as they were already a part of this article, which I have been preparing over the past year. I encourage the reader to refer to Paletta’s article, which was in the May issue of Advanced Aquarist Online. Comparing the articles one can see that I place a greater emphasis on the connection between biological filtration and old tank syndrome.
But first, “benign neglect” The steering issues.
Slow decline in alkalinity
Beginning reef aquarists may not realize the importance of maintaining high alkalinity levels, but old time aquarists who know better also may get lazy and allow the alkalinity to slowly drift downward. This can lead to a gradual decline in the growth of corals and coralline algae, and an increase in the amount of nuisance algae such as Derbesia. When this decline in alkalinity is combined with a rise in phosphate level, an aquarium featuring live corals may shift to an aquarium featuring algae in a short period.
The downward drift in alkalinity occurs because of a few factors. If the addition of calcium and alkalinity is constant as aquariums age, then the trend will be for both to decline over time as calcifiers increase their mass. Eventually, the demand slows or is tempered, however, because the lower sections of corals may die as they become shaded by their own growth. It is a simple matter to periodically monitor the alkalinity level with an alkalinity test kit, and adjust the level or supplementation regimen accordingly. The alkalinity boosting effect of a calcium reactor depends on the maintenance of the calcareous media, which gradually disintegrates and changes the flow characteristics within the reactor. Periodic maintenance is necessary to keep the reactor functioning optimally.
Due to the additions of food, there is a net accumulation of phosphate in the sediment of all aquariums, and an accumulation of phosphate in the water in poorly managed aquariums. The accumulation of phosphate in the sediment is not generally a problem, since it is not very soluble. There is also an accumulation of phosphate on and within live rocks. The use of animals that turnover the sand bed is helpful in keeping it from plugging up with phosphate-rich detritus, and the use of wave makers and other flow devices to physically force the detritus from the rockwork and sand bed can keep the substrata in a healthy condition. The phosphate is in part bound up in this detritus as small particles of calcium phosphate and as various forms of organic phosphate. Some of the precipitated phosphate is deposited on exposed rock surfaces, but these are soon coated by coralline algae, sponges, and other living creatures, in effect isolating the phosphate from the main body of water.
While precipitated phosphate is not problematic, as it is a feature of all aquariums and natural ecosystems, the accumulation of inorganic and organic phosphates in the water can contribute to such old tank syndrome “symptoms” as slow coral growth, disappearance of coralline algae and “greening” of the live rock. The use of protein skimming, kalkwasser dosing, algal filtration, and phosphate adsorption media are all effective ways to limit the amount of phosphate dissolved in the water. Biomass harvest of all kinds, not just of algae, but also of corals and bacteria, can be used as a phosphate limiting process (Delbeek and Sprung, 2005).
If there are places in the aquarium where detritus accumulates heavily, periodic siphoning to remove the detritus is a beneficial maintenance procedure, which can be accomplished along with water change. However, it should not involve large scale, tank-wide gravel vacuuming, as is commonly practiced in fish-only aquariums. A small section at a time is all that should be done, because the sand bed is a living community that can be severely disturbed by such activity.
Old tank syndrome symptoms can be an effect of insufficient water changes. While it is true that we can manage the accumulation of nitrate and phosphate, remove water staining organic substances, and also maintain calcium and alkalinity without performing a water change, the ionic composition of the water still drifts with time in an aquarium containing so much life. Bingman (1998; 1999) discusses this effect, with respect to some major ions, and Fosså and Nilsen (1996) show the changes that occur with some select ions in aquariums maintained with various water change regimes, including no water change. Water changes of approximately 10 to 25% monthly help to limit the long-term change in the ionic composition of seawater in a closed system aquarium.
Flow reduction in pumps
The loss of water velocity in the aquarium with time due to the reduced output by the main circulatory pump(s) is universal. It has numerous causes, and is a cause for reduced efficiency of biological filtration within substrates, not to mention reduced gas exchange.
Flow reduction due to growth of corals
Not only do pumps lose output capacity over time, the velocity of water flow in aquariums is further reduced by the growth of corals, whose branches and polyps dampen the flow. In this case, either upsizing the main or circulation pump(s) in the aquarium or pruning coral heads is required. For this reason, creating more open aquascapes is recommended, with less rock and coral, allowing the corals to grow at a more natural density, and allowing for more flow around each colony.
Light reduction by growth of corals
The coral growth not only blocks water flow, it also reduces the light reaching the substrata. This shading effect alters the rate of photosynthesis, the pH swings, the generation of oxygen, and the consumption of carbon dioxide, among other things. This can result in a decrease in health of shaded corals, as they gradually don’t receive sufficient light, especially considering the gradual decrease in lamp light output over time.
Aside from the “steering issues,” there are other possible factors involved with long-term changes in our aquaria.
Heavy metal accumulation
Shimek (2002a,b, 2003) proposes that due to artificial seawater mixes with high metal concentrations, and high inputs of food containing trace metals, our aquariums are like toxic waste dumps, and the result is an accumulation of heavy metals in closed systems. He recommends periodic replacement of rocks and sand as a maintenance routine. Sekha (2003) and Harker (2003b, 2004a,b) provide evidence to dispute the toxicity of metals that accumulate. It is not known whether accumulation of any metal is related to long-term declines in reef aquarium vigor. If it were, one would expect all reef aquariums to suffer from the same effect after a similar period. They don’t, possibly due to differences in maintenance regimes, or the hypothesis is incorrect.
Accumulation of allelopathic coral metabolites
Borneman (2001, 2002d) discusses potential effects of various allelopathic metabolic products from hard and soft corals that leach into the water, and provides a list of supporting reference material from the scientific literature. This concept has been visited before (see for example, Wilkens, 1990; Delbeek and Sprung, 1994). While it is true that abundance of soft corals in an aquarium can limit the vigor of stony corals and vice versa, the problem is explaining how this effect can become chronically worse as an aquarium ages. We think of it as an issue of the composition of corals in the aquarium, and we know this presents an interesting challenge when planning this composition. However, if it were just a matter of keeping the mass ratios of certain creatures within certain limits, we could define this more precisely. It is certainly possible that as the mass of the soft or stony corals increases with time, it can have a progressively stronger allelopathic effect on other stony or soft corals simply because as the mass increases it is releasing a greater quantity of compounds. Borneman (2001, 2002d) hypothesizes that the effects of this chemical warfare may be the cause of periodic unexplained losses of corals. Since there is so little known about the fate of these compounds in closed system aquariums, it is hard to verify or dispute such a claim. It is likely that allelopathic compounds causing all kinds of problems could be an unpredictable artifact of the closed system environment, but don’t let that scare you! The long-term success of reef aquariums of all sizes is evidence that this kind of problem is not the rule.
I believe the reason for our success despite the chemical warfare is the most significant fact to look at for an explanation of what old tank syndrome really is. In the several lectures I gave in 2005 and 2006 on this topic I made the connection between old tank syndrome and new tank syndrome. New tank syndrome is described classically as being caused by an insufficient capacity of biological filtration. If that is the case, then could old tank syndrome also be caused by the same thing? If so, how is that?
The development of bacteria populations and other fauna within the substrata in an aquarium helps to break down allelopathic compounds quickly, and of course it is possible that protein skimmers, ozone, or activated carbon helps to keep them in check. Why they might intermittently wreak havoc, if in fact they do, is a potential answer to the question of what old tank syndrome really is. The next section, which describes the operation of biological filters, offers a clue to how and why allelopathic compound might sometimes intermittently cause “old tank” symptoms.
In chapter 6 in The Reef Aquarium Volume Three, Charles Delbeek and I outline the way biological filtration occurs in the bottom substrata, and how advective flow of water is key to the process. With time, bottom substrata collect detritus that impedes the advective flow of water thus reducing the input of oxygen, and as the detritus decomposes, it depletes oxygen from the water within the substrata. These combined effects alter the functioning of the substrata, and may contribute to a long-term decline in the health of a reef aquarium. Despite these facts, we do not mean to imply that the aquarist should strive to eliminate detritus! On the contrary, the conditioning of a healthy aquarium involves a certain amount of detritus accumulation that promotes a healthy diversity of life within the substrata. Later on, this accumulation can become excessive, and then it interferes with the proper functioning of the natural biological filter.
Affect on advection and on biological filtration—what’s old becomes new again.
What is biological filtration? Traditionally it is thought of as the management of nitrogenous waste. However, the bacteria in a biological filter bed also digest many sorts of organic compounds, and their function helps to maintain a “healthy environment” in ways that have little to do with the nitrogen cycle. This concept is hardly ever discussed in contemporary biological filtration articles and lectures, but it is well known in the field of “biospherics.” In the early 1990’s I was hired to consult for the Biosphere 2 project, which, if you recall, was a fantastic project wherein a large sealed greenhouse was constructed that contained several of the earth’s major ecosystems, including a rainforest, ocean, coral reef, and savannah. A group of scientists also were contained in this closed system for a couple of years, studying the changes that occurred, in their environment. The maintenance of the air in such closed systems parallels the maintenance of water in our closed system aquariums. Maintenance of oxygen and carbon dioxide levels is probably the first thing you think about, and you would probably imagine that the use of plants in sufficient quantity can achieve a balance. In fact it is not so simple to achieve a balance, but the idea is in that direction. An important air quality parameter that the engineers of this project had to consider was the release of volatile substances into the air by the plastics and other technical components of the structure. This effect is a slight concern for all of us in our homes and offices, but it is not a big concern for us since our buildings are not sealed shut, but breathe and exchange air with the outside. In a closed system the accumulation of these substances in the air can cause serious health effects. The solution to the problem was to pass the air through soil bed reactors. Soil bed reactors are basically damp soil beds through which air is drawn by displacement caused by fans pumping the air out of the space below the soil bed. Plants are grown in the soil, and various types of bacteria and other life colonize the soil. Sounds a bit like a sandbed in an aquarium, no?
What the microorganisms in the sandbed do is assimilate and decompose the various volatile organic substances in the air passing through the soil bed reactor after adsorption of these substances onto soil particles or trapping by the water. To me this feature of soil beds is a fascinating demonstration of the earth’s ability to detoxify pollutants of many sorts, through a combination of physical and biological effects.
Allelopathic affects on bacteria directly
We tend to think of the effects of allelopathic compounds as warfare between this coral and that coral or this polyp or this alga, etc. In fact some allelopathic compounds released into the water also have strong antibacterial properties, so their function may directly influence the bacteria in the filter bed. An intended or unintended effect of the allelopathic compound could be to stop the growth of one or more types of bacteria, and this might change the way the biological filter works.
It is conceivable that a form of life (coral, anemone, alga, sponge, bacteria) could grow in a closed system and produce a compound that prevents the growth of bacteria that break the compound down. However, that effect would be short lived, as another bacteria would soon develop to be able to utilize this compound as food… unless the creature that produced it could change the compound to keep its potency effective, and maintain its presence in the water. The idea that our filter beds can or must evolve with time is possibly a factor in the long term changes we see in reef aquariums. This is aside from the rapid process of biofilm maturation and renewal shown in Spotte (1992).
In the next section I discuss the theoretically possible influence of viruses on bacteria, including bacteria living on corals. Allelopathic compounds in a closed system aquarium likewise might affect the bacteria fauna living on corals themselves, and this affect might cause long-term changes in the growth or survival of corals.
Old tank syndrome symptoms may relate to various types of disease.
It is an ugly fact, but after you’ve achieved a perfect understanding of the mechanics, chemical and biological processes involved in building a reef aquarium, there is still the obstacle of disease that can eliminate any chance of having an aquarium you can sit back and enjoy. Contrary to popular belief, diseases are not caused by having imperfect water quality, though it is true that fluctuating parameters may stress fish or invertebrates and thus promote disease. If pathogens are killing your fish and invertebrates, there may be nothing you can test for to show why your multi-thousand dollar high tech aquarium can’t keep any fish or corals alive. Furthermore, it is almost impossible, certainly not practical, to completely prevent the introduction of pathogens in a reef aquarium system. This means that diseases of various sorts will affect the fish or invertebrates in an aquarium at some point. With proper care in the planning of the aquarium, careful quarantine procedures, and excellent maintenance habits, these events can be prevented to a large extent. It is a common boast made by overconfident aquarists that fish don’t get sick in a reef aquarium because it reproduces the natural conditions so closely that the fish have improved immunity against all kinds of disease causing organisms. While there is a small amount of truth in this statement, it is not true for all kinds of diseases and so must be considered false. Occasionally some kind of disease will affect every aquarium.
Long term puzzling problems with corals
The existence of bacteria that can produce rapid tissue necrosis (RTN) in corals is well known, though not completely understood. More recently, a related phenomenon has been observed by some aquarists, and it is at present not well understood or even widely believed to be a real phenomenon. The effect can be described this way. One or more species of coral in an old reef aquarium suddenly stop growing, after many years of rapid growth. Accompanying the growth cessation there may be reduced polyp extension, and tissue necrosis, slow or rapid. Eventually the species affected become extinct in the aquarium, though other corals in the same aquarium remain healthy and continue growing. After such an event, the aquarium may become immune to this one coral species. All subsequent attempts to introduce fragments of the same coral grown from cuttings of the original specimen, but housed now in other aquariums, prove futile, as the coral quickly succumbs and dies. There is no explanation for this problem, but when obvious parasites like red-acro bugs, protozoans, and flatworms have been ruled out, I believe such mysterious coral trouble could be caused by bacteria. The bacteria involved could act either directly on corals or release toxins in the system, much like the “toxic tank syndrome” described in Moe, (1989). In time, it may affect more than one species of coral, so that the aquarium is unable to house certain species, but is able to house and grow others. It is tempting to blame such occurrences on various introduced toxins or something general about “water quality.” Nevertheless, water changes and even changing all the substrate does not make the problem go away. It really seems as if the aquarium has an immune response to the affected coral(s). There is no name for this situation and no recommendation. The antibiotic treatment method published in Sprung and Delbeek (1997) may help, but so far, no one has studied that possibility. The effect of this problem is impossible to distinguish from anti-growth effects caused by allelopathic substances released into the aquarium water by various corals, plants, and bacteria. It is possible that pathogenic bacteria are not the cause of this problem, and the effects of substances released into the water are. I have no experimental evidence to support my hunch that bacteria can affect corals in a way that parallels Moe’s (1989) toxic tank syndrome. It is just a hunch.
The recent research into pathogenic bacteria that affect stony corals lends greater credibility to the existence of this kind of phenomenon, and one day it may lead to an explanation for why this problem and similar bacteria-caused diseases sometimes occur in aquaria. For example, Ben Haim et al., (2003) found that bleaching in Pocillopora damicornis resulted from an attack on the zooxanthellae by the bacteria Vibrio coralliilyticus, whereas bacterium-induced lysis and death of coral tissue are promoted by bacterial extracellular proteases.
Probiotics, predatory bacteria, and bacteria phages
Some hobbyists have been experimenting with adding cultures of non-pathogenic bacteria to their aquarium in an attempt to out compete pathogenic bacteria and reverse RTN. Probiotic application of cultured bacteria for controlling the populations of other bacteria might thus become a maintenance procedure of value for reef aquariums. In another variation of this idea, Stolp and Starr (1963), Shilo (1966), Shilo and Bruff (1965) describe the bacteriolytic effects of the bacterium Bdellovibrio bacteriovorus. Aquarium author Frank deGraaf (1968) proposed that this predatory bacterium might control bacterial populations in nature and it might be possible to apply this effect to aquariums to control pathogenic bacteria. This application may be invaluable in the control of pathogenic bacteria that affect corals and fishes, but it remains to be tested.
The aforementioned problem with stony corals being attacked as if they were a foreign body (a so-called immune response) may be caused by the loss of protective bacteria fauna on the coral surface due to a virus. Bacteriaphages, as such viruses are called, may target specific bacteria fauna normally living on a coral’s mucus, resulting in a reduced resistance to other sorts of bacteria that can then invade the cleaned space. They may also be directly involved in triggering the virulence of some bacteria. Bacteriaphages are extremely abundant in the ocean. The average milliliter of seawater contains over 50 million of them (Fuhrman, 1999)! No study has been undertaken to look at what happens to them in aquaria.
Viruses may also play a role in various coral diseases, and their presence in aquaria has not been studied. Wilson et al. (2005) observed virus like particles (VLPs) in zooxanthellae and host coral tissue. They noted that tissues from heat shocked corals had abundant VLPs spread throughout the sections, while in controls that were not heat shocked the VLPs were more difficult to find. They have a working hypothesis that temperature shock induces latent viruses in zooxanthellae. Furthermore, the authors note a morphological diversity of VLPs, suggesting that a wide variety of viruses may infect corals and zooxanthellae. These observations may support the hypothesis regarding bacteriaphages, and it may also point to other viral controlled conditions affecting corals in nature and in captivity.
It is really hard to summarize all of the directions I have taken with this article! Since the potential causes of old tank syndrome are many, it is an area that is ripe for research. From a more practical application for aquarium hobbyists, the ideas about maintenance and disease issues presented here are intended to give an aquarist some checkpoints to rule out.