When we started keeping corals over forty years ago, our tanks were often referred to as “minireefs”, with the premise being that our tanks were like a chip off the old reef and were just small reefs in a glass box. Over time a realization occurred that while our tanks have corals, fish and live rock, just like a natural reef, in reality they are significantly different than a natural reef. And there is not just a single reason, but a multitude of reasons. First is the biomass to water ratio. Unlike a reef, which is encircled by an almost infinite amount of water, our coral and fish collections have a very high biomass relative to the amount of water surrounding them in our tanks. This difference is significant for many reasons. In the ocean the nutrient levels are exceedingly low, almost undetectable, but if we try to mimic that in our tanks the corals often fail over time. The reason for this is that unlike in the ocean where the corals undergo an almost constant water change and as a result have developed the capacity to take up these critical nutrients almost constantly, in a reef tank this is not the case. Conversely, if the nutrient levels are kept at too high of levels in our tanks, algae thrives. For this reason, we have learned that there is a small “sweet spot” where nutrient levels are adequate to keep our corals thriving, but at the same time limiting to algae growth. However, unlike a natural reef, where the levels are relatively the same on most reefs, in our tanks this varies from tank to tank, and it changes over time as corals and tanks grow and age. This is also why the fallacy of trying to keep all of the levels of not just nutrients but all factors the same as natural seawater often does not produce the results and success one would expect.
This same effect is true for the major elements and minor trace elements as well. While the natural seawater levels of such elements as calcium, magnesium, alkalinity and others is well known and established, trying to maintain those levels in closed systems requires frequent additions of these compounds with there being trade-offs and complications with these additions. While over the past four decades we have made significant advancements in how we manage these systems, there still is no perfect system, unlike a natural reef, which is a perfect system. In addition to needing to make these additions, over time detritus, waste and others compounds and harmful substances may accumulate in our closed systems. As a result, most of us do regular water changes, not to add things, but instead to remove things that our protein skimmers, mechanical filters and other means of filtration may miss. While the synthetic salts we now use have improved significantly over the years, many still have issues and none, in my opinion, are as good as natural seawater. This is yet another reason why our tanks are so different from natural reefs.
Just as the water chemistry and physical attributes of a reef tank are significantly different than a natural reef, so too is the biology. Starting at the almost cellular level, we
are just now starting to understand how important the microbiome around each coral itself is and how it effects the health of the coral. Unlike on a natural reef where a coral and its microbiome may sit undisturbed for decades and hence be very stable, the corals in our tanks are moved, fragmented and manipulated so that their microbiome is anything but stable. As a result, the health of the coral itself may be compromised, unlike the stable environment corals on the reef have grown accustomed to. Similarly, the microbiome where the corals reside is significantly different in our tanks versus those on a natural reef. We may add corals from a wide variety of reefs, locations or even oceans, with each having come from a different macrobiome and each adding their own microbiome. What results is a hodgepodge of biomes and only due to the strong constitution of many corals are they able to survive. In addition, we often add unknown bacteria from bottles, probiotics, and foodstuffs full of beneficial and possibly pathogenic bacteria without fully understanding what we are adding or why.
Just as the micro and macrobiome are significantly different on a reef versus a reef tank, so too is the food chain surrounding a reef. On a natural reef there is an abundance of phytoplankton, zooplankton, bacterioplankton and other foodstuffs to provide for the nutritional needs of the corals and other invertebrates on the reef. This is not possible in closed systems, and our understanding of the coral’s nutritional needs is only beginning. In most instances the corals in our closed systems are starving relative to those on a natural reef.
Moving to next level of life on a reef we have the pests and coral predators that have become far more prevalent today than they were when the hobby was starting. While there obviously are coral pests and predators on a natural reef there are also natural predators for each of these pests. That is why when a coral is damaged or irritated by a pest on the reef the entire reef is never consumed as there is something that consumes the pest when their numbers get too high. Unfortunately, this is often not possible in a reef tank, as usually when we encounter a pest we do not know what consumes it and if we do the damage is often so severe that we lose many corals before we can get the pest under control. With the small number of corals we keep in such a relatively closed space, the damage even of one pest can do is rather impressive compared to what is seen on a natural reef. This is why all newly acquired corals should be dipped and quarantined before they are added to a tank, and this too may alter the microbiome of a coral.
Also in terms of biology, most natural reefs contain far more herbivores than carnivores and omnivores. This helps explain why when these are natural levels unlike in our tanks, algae rarely gain a foothold. This is also why even when nutrients are low, from time to time
and algae bloom may still occur unchecked. And unlike in our tanks, there is at least one consumer of virtually e very algae that occurs on a reef so they are never unchecked.
Lastly, a reef tank is unlike a natural reef in terms of the selection of fish and corals. When you dive on a reef the first thing you notice besides how big it is, is how only a few species of coral dominate each section of a reef. Unlike a reef, we may keep dozens of different corals in our relatively small tanks, and if one starts to dominate, we frag it so that it does not kill its neighbors. In similar fashion, in our tanks we often keep a wide variety of fish, not only from different sections of the reef but often from different oceans as well. Again, this is far different from what occurs on a natural reef and its impact on the entire tank ecosystem is only starting to be understood.
While we have come a long way in our understanding and husbandry of corals, fish and other marine life, we still are not close to replicating a natural reef and we need to understand that the two are only marginally related. It is only due to how resilience of most marine life is that we are able to keep these animals in closed systems.