I have to chuckle when I hear someone say that their reef tank looks just like a reef in the ocean, or that their tank is like a small piece of the reef. It may sound like sacrilege to fellow reefers, but despite how much we want to believe this illusion, or how good our tanks look, or even how large some of our tanks are, they simply are not a piece of a natural reef.

Before you think I have lost my mind for saying this please hear me out. I have not come to this conclusion easily, nor am I saying it just to stir up some controversy. But after diving on numerous reefs, keeping and setting up numerous reef tanks for approximately the past 30 years, I have come to appreciate the significant differences between a natural reef and a reef tank.

It is not my intention to dissuade anyone from keeping a reef tank or thinking that what they have does not look like what is on a reef. Rather, I hope to point out some of the differences so that over time we can be more successful. There is not just one factor that makes our tanks unlike a natural reef, but actually a multitude of factors. When each of these factors is considered it becomes clear why our reef tanks are not merely little pieces of natural reefs. Some of the major factors that separate the reef from the reef aquarium include but are not limited to things such as high biomass-to-water ratio, stability, biodiversity, single species domination, nutrient levels, lighting and plankton.

Biomass refers to the amount of living material in a given space. While there is a great deal of biomass on a coral reef, there is also a significant amount of open ocean surrounding the reef in order to support it. When you consider how large the overall ocean is and how much the biomass of the reefs within it occupy, in order for our reef tanks to be comparable, we would need to house a coral fragment in a swimming pool and multiply that by several hundred in order to get the ratio to be close to “natural”.

None of us are going to do that, so instead what most of us do is go to larger and larger tanks to try and improve this ratio. However even in doing so, we typically do not reduce the biomass to water ratio, we simply add more fish and coral to fill in the empty space. Meanwhile, some of us do try for a better ratio by having a sump that holds significant water volume attached to our tanks. Despite our best intentions, this single factor alone makes our tanks, significantly different than a reef.

This lack of having lots of water within our reef tanks also leads to another significant difference: stability. Or should I say instability. When you have a body of water like the Pacific Ocean which contains around 187 quintillion gallons of water, it takes a lot to change any characteristic of the water within it. As a result the organisms that reside in it like corals, fish, plankton, etc., have come to rely on the stability of the water. Most of these organisms are not designed for rapid fluctuations within the water in which they reside.

Unfortunately, despite the many advancements that have been made in both the equipment and our reef tank husbandry, there is still a significant amount of instability within our tanks. While we have gotten better in maintaining greater stability in things such as temperature, salinity and alkalinity, lots of parameters such as pH, oxygen levels, and nutrient levels can still vary widely not only over time, even just during the course of a day.

For anyone who has maintained reef tanks for a length of time, it is very clear that corals do much better in a stable environment than they do in an unstable one. In a similar way, this high biomass to water ratio also leads to our tanks having significantly higher nutrient levels than occur on the reefs. Phosphate, nitrate and dissolved organics are all generally at levels several fold higher than they are seen on a natural reef. However, we have gotten significantly better at maintaining these compounds at significantly lower levels than we used to. Very rarely does one encounter the algae-choked reef tank that was commonplace even just a few years ago. But the levels in our tanks of these compounds are still much higher than on the reef. And again, most of our corals tolerate rather than thrive at these levels.

Biodiversity, or the lack thereof is also significantly different on a reef than it is within our tanks. Despite adding live rock, live sand, and cultures of various reef organisms, our tanks are sorely lacking in the kind of biodiversity that occurs on a reef, especially in regards to microfauna. In addition few of us have any plankton to speak of within the confines of our tanks, which is important because plankton makes up a significant component of the biomass surrounding a reef.

The lack of a constant supply of plankton and microfauna also makes our tanks significantly different than the reef – this constant supply of food allows reef animals to feed almost constantly, rather than in the one or two big meals that we typically provide for our tanks. The absence of having much of the smaller reef animals animals present in our tanks also results in the processes that these organisms provide on the reef to be lacking in our tanks. Many of which we do not even understand. Someday I would love to get some perfect fresh pieces of live rock covered in sponges, tunicates, hydroids and other microfauna and keep it alive just to see what all of the microfauna does. Unfortunately virtually all of the live rock we see today is devoid of any kind of this life, but I think that really would be more like a small piece of the reef.

In contrast to the lack of biodiversity in our tanks, our tanks also tend to lack species domination. That is, unlike on a reef where during a dive one can go over what seem like endless fields of leather corals or Goniopora or table corals, our tanks tend to house many different corals and invertebrates not only in terms of type and species, but also in terms of the zone within the reef from which they are from and sometimes even in terms of the oceans from which they are from. As a result, our tanks are not really representative of a small section of the reef, but rather are what our ideal of a section of the reef should be.

I will admit that I am one of the worst offenders in this regard as there are simply too many beautiful corals to limit my tank to housing only a few. But until we become more inclined to house only a few or a single species of coral in a tank, our tanks will not be close to a natural reef. The corals we house in our tanks are generally much more colorful overall than the corals on the reef.

While diving over a reef you may have to cover an area the size of a football field just to find one or two colorful corals. Meanwhile, in our tanks it is commonplace to find 50 or more colorful corals in less than a square meter. Again this is a major difference between our tanks and the reef. The reality is we are not inclined to simply keep brown sticks, color and diversity is what has driven the hobby to the heights it has now reached.

In the same vein, even though we typically only keep small reef fish, even in the largest of home reef tanks the difference from the reef is quite amazing when you think of it. On the reef a fish may have thousands of cubic meters to call its home, whereas in most reef tank it will be lucky to have a single cubic meter. However it should be pointed out that on the reef a fish has not only competition for food and space, but also a constant threat of being eaten. In this respect a reef tank may be superior in that the life expectancy of a properly cared-for fish or coral may be several times longer than it would be on the reef.

Lastly, despite all of the advances we have made in properly lighting our reef tanks, we still are not even close to what lights the reefs: the sun. I have run thousand watt metal halides, in state-of-the-art reflectors, and brought them outside and even in Pittsburgh, where the sun is not as bright as in the tropics, when the light emitted from these units was compared with natural sunshine it was as if they weren’t even on.

Having said that, the lighting of today and the advancements that are still coming have brought closer to providing light that allows our tanks to thrive. We can now tailor our lights to provide a spectrum that not only enhances our viewing pleasure but also allows the corals to thrive and even bring out colors that are not apparent on the reef. So again our tanks are significantly different than how a reef looks when in sunlight.

Please understand I did not write this to diminish the advancements we have made in the hobby over the last 30 years nor am I saying that our reef tanks are not beautiful. Also without the advancements we have made in the hobby, much of the knowledge we have gained in understanding corals would not have occurred. Compared with what tanks looked like and the limited types of corals and fish we used to keep, the level of advancement from then to now is indeed mind-boggling. Most of our tanks are truly beautiful and teem with interesting life that we could not keep alive 30 years ago, let alone have thrive.

These advancements have even in many instances even allowed a significant number of animals we now keep to reproduce within our tanks. However, we need to understand that despite our best efforts our tanks are not really chips off the old reef block, but rather special captive systems, and we would be better served trying to optimize the conditions for our inhabitant’s long-term success rather than trying to replicate or emulate the conditions and look of the reef.

While we will continue to strive to make our reef tanks closer to their natural counterparts, realistically this is not possible. So rather than trying to think that we have a small piece of a natural reef, it may be more fruitful to understand the shortcomings of our small closed captive systems and try to maximize what we do to grow and reproduce what we have in these closed systems and enjoy and have fun with the fruits of our labor.