How Many Fish Can My Aquarium Hold?

Follow these steps to more accurately estimate the swimming space needs of your aquarium fish.

AFIA  common question faced by all aquarists is: “What size aquarium do I need for the fish that I want to keep?” This article will help aquarists identify potential problems in terms of acquiring a fish that may outgrow their available aquarium resources. It is always better to answer this question before purchasing a new animal than having to deal with a fish that has grown too large for its aquarium.

Most aquarists understand that every aquarium has a biological “carrying capacity,” primarily the ability of the filtration system to render the waste products produced by the animals into relatively nontoxic substances. If the carrying capacity is exceeded, the result is deteriorating water quality (dropping pH, rising nitrate levels, rising ammonia or declining dissolved oxygen concentrations).
Aquarists also know that each aquarium has a “territorial limit,” in terms of how many fish of which species and of what size can be safely housed together before fighting becomes an issue. In some cases, two species will simply never be compatible. In other instances, the level of compatibility will depend on tank decorations, relative sizes of the fish and even each fish’s individual nature.

Once the issues of carrying capacity and territorial limit have been resolved, there is still the question of how much swimming room each fish requires. Many aquarists attempt to determine this need for swimming space based mostly on their own experiences. New aquarists lacking this knowledge have a distinct disadvantage.

What is the result of keeping a fish in an aquarium that does not offer it enough swimming room? The biggest concern with a fish in an aquarium that is too small: it may suffer chronic injury. If a fish cannot comfortably turn in the aquarium, it may bump its mouth or eye on the glass, which can eventually lead to tissue damage, such as a rubbed snout, bulging eyes, deformed fins, etc. If the fish is showing no detrimental signs of being housed in too small of an aquarium, is there even a problem? What about the fish’s psychological well-being? The question of swimming space frequently becomes an emotional issue that cannot readily be resolved.

This article examines the need for swimming room based more on tangible measurements and less on opinion. The process begins by first determining the values for the three variables that need to be used in the calculation:

  1. Expected adult size of the fish,
  2. Swimming habits of the species and
  3. Tank measurement.

Expected Adult Size
Determining the adult size of fish in captivity confounds many people. Just what is the expected maximum size of our fish in captivity? Many aquarists use the Internet site for maximum size records of fish and extrapolate captive-size estimates from that data. The problem with this is two-fold. First, FishBase lists the maximum size recorded, not normal adult size. And second, many fish do not grow as large in captivity as they may in the wild. 


In order to test whether FishBase reflects the normal maximum captive sizes of adult fish, I used specimens at a public aquarium as a comparison. The fish in my study group had reached their full captive adult sizes, and it was found that this ranged from 45 percent to 91 percent of the maximum FishBase size for each of the species. The average was 66 percent. So if you use FishBase to estimate how large a fish will grow in an aquarium, it is more accurate to use two-thirds of that value as the captive maximum size. Aquarists wishing to be a bit more conservative in their calculations could use 80 percent of the stated maximum value. There are a few exceptions to this process; some small tetras, gobies, clownfishes and other species that may live years longer in captivity than they would in the wild will sometimes exceed the maximum wild sizes listed on FishBase.

Swimming Habits
The next step in the process is to determine the swimming habits of the species in question. Obviously, anglerfish utilize less swimming room than surgeonfish. Three basic swimming habits were selected:

  1. Fish that spend at least some of their time stationary on the bottom.
  2. Species that maneuver in and around underwater structures.
  3. Active, open-water swimmers.

For a list of fish that fit these categories, see the “Swimming Habits” sidebar.

In many cases, you can tell which of the three categories a fish will fit into based on its tail shape. Fish with lunate caudal fins (e.g., jacks, tuna) are more typically open-water swimmers that cannot turn well. Fish with crescent-shaped tails (e.g., tangs, Moorish idols) can maneuver a bit better but still can swim fairly rapidly in a straight line. Fish with squared-off tails (marine angelfish and discus, for example) are slower but more maneuverable. Fish with soft, rounded caudal fins (e.g., percula clownfish and dottybacks) are slower swimmers, but they are highly maneuverable.
Of course, there are some exceptions to these rules. Some adult male anthias, cichlids and wrasses develop streamers on their caudal fins that make them look like open-water swimmers, but these are really “all show and no go.” Other fish, such as wrasses and parrotfishes, do not use their caudal fins as their primary method for propulsion, so the shape of that fin cannot be used to predict the swimming needs of those fish.

Aquarium Size
The final of the three variables is aquarium size. Since fish typically swim in three dimensions, it was initially thought that using the aquarium’s length multiplied by its width and by its height would work as a way to compare different aquariums. It turns out that if this is done, one actually ends up comparing aquarium volume to the linear measurement of the fish. This does not work because as the fish length increases linearly, the tank volume increases by the cube, and the ratio of these numbers is meaningless. It is best to discard height and just use length plus width. To obtain the proper measurement, take the width of the aquarium’s open-water area and add it to the length of the longest open-water stretch (not counting space taken up by decorations).

Complete the equation by taking the open-water tank length plus open-water tank width and dividing it by maximum fish length. This result is expressed as a ratio then compared to a data set for fish from current aquariums that are being properly operated. For each of the three swimming habits, an absolute minimum and a preferred minimum ratio was determined (listed on the “Minimum Ratios” chart).
To illustrate this, take the real-life example of a 10-inch-long bignose unicornfish (Naso vlamingii). FishBase lists the maximum size of this species as 23.4 inches, so multiplying that by two-thirds (0.66) gives a predicted maximum captive size of 15.5 inches. This fish is in an aquarium that has an open-water area that is 70 inches long by 50 inches wide, for a total of 120 inches. At the fish’s current size, the ratio would be 1-to-12, slightly above the preferred minimum ratio. If the fish grows to the predicted maximum size, the ratio drops to roughly 1-to-8, slightly below the absolute minimum range for an active swimmer likeN. vlamingii.

Sharks: A Special Case

Sharks have long held a fascination for many home aquarists. Some species, such as reef sharks, are obligate ram ventilators, meaning that they must swim continuously in order to breathe. These fish obviously require huge amounts of swimming space. For example, a 4-foot-long Pacific blacktip reef shark requires an absolute minimum of 12 feet by 12 feet of appropriate swimming space (a lot of open water with few obstructions). Because large aquariums are so expensive, aquarists wishing to keep sharks in their homes often try to make do with the smallest tank possible — which is a mistake.
With other sharks, swimming space may not always be the biggest concern. Many sharks sold to home aquarists (e.g., leopard sharks, smoothhounds and horn sharks) are cool-water species that do not thrive at tropical temperatures. Avoid keeping sharks in home aquariums, with the exception of epaulette and coral catsharks, which are tropical species that do not often swim and stay relatively small (less than 3 feet).

Some Warnings
Bear in mind that this method only estimates the swimming space requirements for a single fish in a given aquarium. It does not calculate the biological carrying capacity or territorial limits of an aquarium. It also does not factor in the need for increased swimming room when multiple fish are utilizing the same space. This means that if the formula suggests you can maintain a surgeonfish in a 100-gallon aquarium, this will not necessarily be the case for two surgeonfish to be housed in that aquarium. This is where extra research comes in. If an aquarist learns that a fish will be “messy” (such as goldfish), this means there will be more strain on the biological carrying capacity of the aquarium; research also allows the aquarist to learn more about the aggressiveness and behavior of the fish in question before deciding how much space multiple fish will need. Some other fish have very specific requirements and do not fit the mold. (For more information, read the “Sharks: A Special Case” sidebar.)

Some aquarists have suggested that this formula does not apply to very small fish. Take the case of a 1-inch-long zebra danio (category 3). This active fish requires open-water swimming space of 10 times its length, perhaps a 7-inch-by-3-inch space. This strikes most home aquarists as being too small. However, this is an illusion created by personal bias. For example, the zebra danios in the previous example have proportionally just as much room to swim as a 6-inch-long wrasse does in a 48-inch-by-12-inch open-water space, say a 55-gallon aquarium, which would not seem too small to most aquarists.

Can fish be “happy”?
Find out the author’s thoughts at
and get the author’s references for this article at

Readers must understand that this method gives the suggested minimum size for an aquarium needed to house a particular species — it was not designed as a way to validate aquarists trying to overstock aquariums or to keep fish in an aquarium that is too small. It does, however, offer a realistic way to determine the minimum swimming-space requirements of fish and thus enables aquarists to offer their fish the best possible captive environments to live, grow and reproduce. You cannot go wrong by offering your fish more swimming space than these calculations show, but fish can quickly run into trouble if they are given less swimming room. AFI

Jay Hemdal has worked in public aquariums for two decades and is presently the Curator of Fishes and Invertebrates at the Toledo Zoo. He has written the books Aquarium Careers and Aquarium Fish Breeding, as well as more than 100 magazine articles.



Article Categories:
Fish · Health and Care