Among the first considerations in planning the construction of any aquarium system is tank size. This is certainly understandable, as most of us have to work within some kind of limitations with regard to space or budget. For those restricted primarily by the amount of available floor space (and perhaps working space) for the aquarium itself, the decision can be made rather quickly and simply with the use of a common tape measure. On the other hand, for those who must work within the restrictions of a budget (even a generous one), the answer is far less self-evident. This is particularly so for the beginning reef hobbyist, who may not yet fully realize what kind of commitment they are getting themselves into.
Reef aquaria—regardless of size—are costly. Adequately equipping and maintain them is even more costly. Therefore, being able to set (and stick to) a monthly budget is perhaps even more important as meeting ones budget for the initial purchase and installation. Just think about that for a moment. Month after month, week after week, day after day you will be feeding the fish, performing water changes, changing light bulbs, replacing filter media, etc. The size of the aquarium will have a direct influence on how much these ongoing expenses will actually cost. Too often (typically after a windfall like a tax return), an aquarist will purchase a tank of the largest size that they can “afford” at that time, only to watch its inhabitants languish over the following months as the keeper is incapable or unwilling to spend what is required for the regular upkeep. Aquarium systems of this type, generally equipped with substandard lighting, filter components, etc., are a staple of garage sales and local classified ads.
These kinds of cases are especially unfortunate because they involve the maltreatment of live, often wild, animals. In short, a healthy reef aquarium is much preferable to unhealthy reef aquaria of any size. With this in mind, be sure to include all necessary components of a conventional reef aquarium when establishing the initial budget. This means that certain pieces of equipment with a proven high level of utility (e.g. protein skimmers and UV sterilizers) should not be eschewed as “optional.” Furthermore, you should buy only quality equipment from a reputable manufacturer. You might wonder if you really need these things if you just have a small system; the short answer is that you will certainly need them because your system is small. Will this cut deeply into the budget allotment for the tank and cabinetry? It absolutely will, and that is the point. And just the same, make sure that there is room in your monthly budget to keep up with regular maintenance as well as for emergencies.
Undoubtedly, a responsible and well-informed aquarist can successfully keep many reef species, including stony corals, in a smallish aquarium system. The growing popularity of so-called nano tanks (often defined as systems under 15 or 20 gallons) is certainly evidence of that. But, again, it is typically far less problematic to house these delicate creatures in larger aquaria. For one, it is much simpler to install a sump/overflow system in a tank that is at least, say, 55 gallons in size. An overflow will help to eliminate films on the water surface that inhibit gas exchange and reduce light penetration. Additionally, the sump itself can significantly increase the total aquarium system water volume and, thus, level of stability. Yet another factor here is the transfer of thermal energy from equipment such as lighting. Remember, many soft and stony corals alike require very strong illumination; even some of the most efficient types of aquarium lighting fixtures generate a considerable amount of heat, to the detriment of many species. This can create the need for a chiller, which might cost more than the rest of the entire aquarium system.
These days, many smaller aquarium systems can be purchased as “plug-and-play” units that incorporate most, but seldom all, important components. These can be a great choice for hobbyists who for reasons of either space or budget restrictions must opt for a very small tank. For one, the components are of course made to fit the unit; trying to fit a mix of components from different manufacturers into a nano-sized tank can sometimes be a nightmare due to the tight spaces in their filter compartments or sumps. And, by purchasing all of the components together, one has a chance of obtaining them at package-deal pricing. Do note, however, that the quality of these nominally “all-in-one” systems varies greatly, and that many are downright unsuitable (despite the manufacturer’s claims) for most reef aquarium species.
To be clear, there really is no formula for the perfect or absolute minimum size of reef aquarium. There are simply too many factors at play. Number of animals (e.g. bioload), species composition, even the feeding habits of the keeper can be at play here. The best way to ensure success (i.e. provide a consistently healthy captive environment for your livestock) is to purchase the largest system that you can afford to properly equip and maintain over time.