A reef tank is a reef tank is a reef tank, right? Well, not necessarily. While we could certainly call every marine system with liverock, corals, and a wide variety of animals that swim and crawl a reef tank, there is a fair amount of differentiation between systems based on their size. Obviously we don’t keep tangs in 20-gallon tanks, and there would be little motivation to house a diminutive trimma goby in a 210-gallon reef, but there’s a lot of gray area with respect to space requirements for many of the available livestock options out there.
But before we can get onto the topic of stocking, we need to first define some ambiguous terms. The reef aquarium categorizations of “micro,” “nano,” and “pico” get used all the time but with little reference to what they actually mean, so I’d like to take a moment to try to assign some basic parameters to these words.
Porcelain crab. Photo by Alex Rose
It’s easy to forget that these prefixes have standardized meanings in the world of metric measurement when they’re used interchangeably in the aquarium trade so often. From a literal standpoint, micro (10-6) means millionth, nano (10-9) means billionth, and pico (10-12) means trillionth, leading us to assume that a micro-reef is larger than a nano-reef which is larger than a pico-reef. Of course, this analysis only tells us their average size relative to one another, so I’m going to make an effort to assign some actual tank volumes to these arbitrary terms. It seems as though micro refers to tanks from 35-55 gallons, nano indicates systems from 6-34 gallons, and pico means aquariums less than 6 gallons. It is important to remember that no specific divisions have been made or agreements come to about these definitions, but that these tank volumes have been chosen based on what seems to be general consensus on several forums. At one point, the industry standard for a nano-reef was that the tank had no dimension larger than 2 feet. We can extrapolate this to mean that the largest nano-reef could measure 2ft x 2ft x 2ft, giving it a total volume of 55 gallons, but by this definition, our micro-reef would now be our nano-reef. Trying to accurately categorize these terms has been described as “shooting at a moving target,” so for the purpose of discussion, let’s stick with the parameters presented above for now.
The wide size range encompassed by the term nano-reef presents some difficulties when considering the stocking of these small systems. There’s a huge difference between what can be housed comfortably in a 34-gallon reef compared to a 6-gallon one, so as aquarists we really need to exercise our best judgment when it comes to what will be living in our aquariums. As is always the case, research is imperative in order to ensure that a given tank will be able to provide the desired animals with their necessary habitat requirements. It is also important to remember that small systems are far more susceptible to fluctuations in water parameters, so water quality maintenance is particularly critical as is proper stocking density.
Considering the plethora of options we have for stocking a nano-reef, the remainder of this article will focus entirely on the much smaller subset of animals that can live in a pico-reef. My next piece will be a follow-up discussing only nano-reefs and their potential inhabitants.
The livestock options for a pico-reef are fairly limited. Even in the largest pico-reef (5g), a single fish may be pushing the limits of the bioload that can be safely handled. Of course, diligent water changes are absolutely necessary and even temporary lapses in maintenance can prove fatal for tank inhabitants. Some aquarists discourage keeping fish in such small systems entirely and suggest only housing corals and invertebrates, but it is certainly possible to have a healthy, vibrant pico-reef with a fish. The only vertebrate tank inhabitants that I can recommend for an aquarium like this are gobies, shrimp gobies in particular. They are ideal because in nature, their territories are tiny and they can consequently be content in the close quarters of a pico-reef. Shrimp gobies are fantastically interesting fish to keep that will provide their aquarists with hours of observational entertainment. While they can survive without a commensal shrimp present, it is the nature of this relationship that makes them so fascinating. There are approximately 130 species of shrimp gobies from 20 different genera that have symbiotic relationships with pistol shrimps in the genus Alpheus. These pistol shrimps have extremely poor vision and depend heavily on the presence of an associated goby for daily activities. The shrimp is responsible for digging and maintaining a tunnel that it shares with the goby, and the goby serves as a lookout for potential danger. The shrimp will almost constantly remain in contact with the goby through its antennae, and both will dart back into their tunnel if threatened.
There are many shrimp goby species that get too large for pico-reefs, but quite a few also stay small enough for this type of tank. In particular, gobies in the genera Stonogobiops and Ctenogobiops stay diminutive and rarely get larger than 2.5 inches as adults. The tiny Spikefin Shrimp Goby (Discordipinna griessingeri), which is only an inch in length as a mature adult, is quite possibly one of the most beautiful fish in the trade, regardless of size. All shrimp gobies and their commensal shrimps live in burrows, so they must be provided with the proper amount of substrate in order to construct a tunnel.
Gobies in the genera Trimma (e.g. Red Spotted Goby), Elacatinus (e.g. Neon gobies), Eviota (e.g. Gold Neon Eviota Goby), and Gobiodon (e.g. Clown gobies) typically remain 2 inches or less and can make great pico-reef inhabitants, but care should always be taken to research the fish before purchasing it.
Invertebrates are often the best options for reefs under 6 gallons because they are content with small spaces and produce far less waste than fish do, meaning that the bioload of inverts is substantially lower than that of fish. Small shrimps, crabs, and snails are perfect pico livestock choices. Sexy Anemone Shrimp (Thor amboinensis) and Porcelain Crabs (most commonly Petrolisthes spp.) are particularly attractive spineless options that are both colorful and active in a small reef, but there are many choices available. It is best to avoid species that associate strongly with SPS corals, as it is nearly impossible for anyone but an advanced aquarist to provide them with the proper habitat for optimal living conditions in such a small volume of water.
Alex Rose is a biologist (BS and MS Biology), diver (PADI Divemaster), musician, underwater photographer, and lover of all things aquatic. Her driving goal is to find ways to protect our world’s coral reefs through diving, writing, education, and the establishment of a sustainable marine aquarium trade.