Estimating the Need for Swimming Space for Aquarium Fishes

This neon fusiler is an open water swimmer that would belong in category three.jpg
Estimating the Need for Swimming Space for Aquarium Fishes

Jay Hemdal

For those not wishing to read the entire article, here is the basic formula for calculating the swimming space needs for active swimming species (tangs, wrasses, fusiliers, etc.)

Look up the maximum adult size of the species at Multiple that number by 0.80 This is the expected maximum size for this fish to reach in a home aquarium. Call this variable A

Measure the open water space of your display tank’s length and width and add them together. Depth isn’t important if it is greater than 12", and areas filled with corals and rock must not be measured, and any water depth less than 12" also needs to be ignored. This is variable B

Create a ratio with those two numbers. B divided by A. If this value is less than 8, you really should not keep this fish in that aquarium as it reaches its full adult size. If the number is greater than 10, this fish will likely be comfortable in this aquarium for its life span.

Please note: This formula only looks at a single fish, and its ability to determine if an aquarium is large enough for it. Most aquariums house multiple fish. This equation cannot take that into account. If you try to apply this for a large number of fish, there will be territorial limits that come into play, so better to err on the side of caution, and give groups of fish much more space.

A 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. Remember that it is always much better to answer this question for yourself before a new animal is purchased, than to have 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 for the filtration system to render the waste products produced by the animals into relatively non-toxic substances. If the carrying capacity is exceeded, the result is deteriorating water quality such as dropping pH, rising nitrate levels and in extreme cases, rising ammonia and/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 (Hemdal 2006). In some cases, two species will simple never be compatible. In other instances, the level of compatibility will vary depending on tank decorations, relative sizes of the fish, and even the fish’s individual nature.

Once the issue of carrying capacity and territorial limit have been resolved, there is still the third question of “how much swimming room does each of the fish require in order to remain healthy?” In the past, aquarists attempted to determine this “need for swimming space” based mostly on their own experiences. New aquarists lacking this knowledge would therefore be at 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 fish in too small of an aquarium is that they may suffer chronic injury. If the fish cannot comfortably turn in the aquarium, it may bump its mouth, or eyes. Done enough times, this can lead to tissue damage; rubbed snouts, 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; tank measurement, body style of the species, and the expected adult size of the fish.

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 is two-fold, first the FishBase records are for maximum sizes 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 the ability of FishBase to reflect the normal maximum captive sizes of adult fish, specimens at a public aquarium were used as a comparison. The fish in the study group had reached their full captive adult size and it was found that this ranged from 45% to 91% of the maximum FishBase size for each of the species. The average was 66%. So if people are using the FishBase maximum size records 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% of the stated maximum value. There are a few exceptions to this process; some small tetras, gobies, clownfish and other species that may live years longer in captivity than they would in the wild, will sometimes exceed the maximum wild sizes listed in FishBase.

The next step in the process is to determine the body style of the species of fish in question. Obviously, anglerfish utilize less swimming room than do surgeonfish. Three basic body types were selected; fish that spend at least some of their time stationary on the bottom (category 1), species used to maneuvering in and around underwater structures (category 2), and active, open water swimmers (category 3)

Category 1
Stationary Species
Category 2
Maneuverable Swimmers
Category 3
Open Water Swimmers
Corydoras catfishCichlids (most)Fusilers
Dwarf lionfishClownfishJacks
Eels (divide length by 3)DamselfishMoorish Idol
GobiesDiscusPangasiid catfish
Lungfish (2/3 length)Marine angelfish or butterflyfishParrotfish
Pipefish (divide length by 2)Poecilids – livebearers
Cyprinids – minnows
Pelagic sharks
Seahorses (use ½ body height)Rasboras
Rainbowfish / Atherinids
ScorpionfishStingraysSurgeonfish / tangs
Suckermouth catfishTriggerfishWrasses (most)
Suggested examples of fish for each of the three swimming style categories

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 x height x width 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 measure of the fish and this does not work because as the fish length increases linearly, the tank volume increases by the cube, and the ratio of these numbers are meaningless. Since most aquariums have a height that is proportional to their length and width, it was decided to remove the former from the equation. Therefore, the length plus the width value is linear, and can be directly compared to fish’s length. One final factor is that many aquariums, (especially reef tanks) have many solid features which obviously reduce the available swimming room for the fish. Therefore, to obtain the proper measurement, the aquarist would take the width of the aquarium’s open water area and add it to the length of the longest open water stretch, thus not counting space taken up by decorations. Cylindrical aquariums are a special case, as they allow fish to swim constantly. As long as the fish are small enough to be able to swim the diameter of the cylinder, these tanks are essentially large enough.

The basic equation is: Open water tank length + open water tank width / maximum fish length. This result is then expressed as a ratio. The ratio was then compared to a data set for fish from currently properly operating aquariums. For each of the three body styles, an absolute minimum and a preferred minimum ratio was determined:

Body style 1 – Sedentary fishes:

Absolute minimum ratio 1:3.5
Preferred minimum ratio 1:5

Body style 2 – Maneuverable swimmers

Absolute minimum ratio 1:5
Preferred minimum ratio 1:7

Body style 3 – Active swimmers

Absolute minimum ratio 1:8
Preferred minimum ratio 1:10

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 .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 1:12, slightly above the preferred minimum ratio. If the fish grows to the predicted maximum size, the ratio drops to 1:7.8, slightly below the absolute minimum range for an active swimmer such as this species.

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, it does not mean that this will necessarily be the case for even two surgeonfish to be housed in that aquarium – this information about multiple fish still needs to be determined from other sources or personal experience.

Some aquarists have suggested that this formula does not apply to very small fishes. Take the case of a one inch long zebra danio. This type 3, actively swimming fish requires a preferred minimum of open water swimming space of ten times its length – perhaps a 7” by 3” space. This strikes most home aquarists as being too small. However, this is an illusion created by personal bias – that zebra danio has proportionally just as much room to swim as a six inch long wrasse does in a 48” by 12” open water space, say a 55-gallon aquarium – which would not seem too small to most aquarists.

You 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 over-stock their aquariums or to keep fish in too small of an aquarium. It does however offer a realistic way to determine the minimum swimming space requirements of fish, and thus adds to the ability for aquarists to offer their fish the best possible captive environment in which 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.

Happy Fish?

When you hear somebody say, “you must do this or that” in order to keep your fish happy” or “your fish simply cannot be happy in that size tank” what do they really mean? Are they using the word “happy” in place of “healthy” or “proper well-being”? Or do they really mean to imply that true happiness can be identified and then quantified in fish? People must be mindful not to project human emotions to animals that do not have the capacity to feel them.

An animal rights advocate may look at a single fish in a tank and exclaim, “how unhappy that fish must be, all alone in that small tank”. I suspect that in these cases, they mean not the fish’s well-being and health, but that it is truly unhappy in the mammal paradigm of how the word is generally used in English.

If somebody says, “hepatus tangs need at least a 150 gallon tank in order to be happy” they are not using the word in place of “healthy” if that same tang can be kept healthy and problem free, long term, in a 75 gallon tank. They are then adding an extra layer to the definition, one that cannot be measured or quantified. I’m not condoning keeping fish in small aquariums, I’m just opining that you cannot formulate tank size requirements based on intangibles such as the “happiness” of a fish. If your opinion is that this example fish is so much happier in the 150 gallon than the 75 gallon, that the smaller tank becomes a non-starter, then the fish would be whole orders of magnitude "happier" in the wild, and shouldn’t be kept in an aquarium at all.

The following is my working definition of “appropriate aquarium husbandry”, and I say this equates to "happy" for a captive fish:

If the fish shows no signs of chronic disease or abnormality, exhibits normal feeding and reproductive behaviors and most importantly, exhibits a normal lifespan compared to that of wild counterparts (minus the predation wild fish incur of course!), then there is no other metric we can use to determine if a certain suite of husbandry techniques are suitable or not.


Froese, R. and D. Pauly. Editors. 2010. FishBase.
World Wide Web electronic publication.

Hemdal, J.F. 2009. Mini-Aquariums. 144p. BowTie Press, Laguna Hills, California

Hemdal 2009. For Good Measure - Calculating the Swimming Space Requirements for Fishes in Aquariums. Practical Fishkeeping Magazine. Issue 7, July, 2009

Hemdal, J.F. 2006. Advanced Marine Aquarium Techniques. 352pp. TFH publications, Neptune City, New Jersey

Mohan, P., Hemdal. J.F., and Loiselle, P. 1997. Tropical lunkers. Aquarium Fish 9(7):60-65.
About author
Jay Hemdal
Jay Hemdal has kept aquarium fish since he was four. He set up his first marine aquarium in 1968 when he was nine years old. He later worked part time for many years at various local retail pet stores and fish wholesale companies while he was living at home and then during college. After graduating from college with a degree in aquatic biology, he managed the aquarium department of a large retail pet store for five years until 1985, when he was hired as an aquarist/diver (and later department manager) for a large public aquarium. In 1989, he accepted the position of curator of fishes and invertebrates for another public aquarium, where he remains today. Jay has written over 200 articles and papers as well as seven books since 1981.

Jay has also written for the following publications.

Advanced Aquarist Magazine
Aquarium Fish International
Aquarium Frontiers
Comparative Parasitology
Drum and Croaker
Freshwater and Marine Aquarium
International Zoo Yearbook
Journal of Aquariculture and Aquatic Sciences
North American Journal of Aquaculture
Progressive Fish-Culturist
Today's Aquarist
Tropical Fish Hobbyist
Zoo Biology

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This a great informative article.
Thanks Jay
So this is pretty interesting for sure. It makes me think that a 90 gallon tank really doesn't hold a role in Active Swimmers.

Take a yellow tang who gets up to 8" long. The adjusted size is 6.4" The max area for a 90 gallon (48x18) is 64. That would obviously be adjusted down. So essentially you're best-case scenario is right at the recommended, but again that's an empty tank.

Does that mean you shouldn't get that tang, or you should be prepared to part with it before it reaches full maturity?
This is far and away one of the best articles here. When one considers that fish are taken from lakes and left to die in 'creels' - or that multiple of the fish we keep in tanks are widely killed and eaten in fish markets around the world. To ME - the topic of fish happiness has a lot more to look at than the average reef tank
Great info. I definitely appreciate your view on the use of the word 'happiness' in this hobby; I 100% agree!

Thank you for taking the time to write this article!
As usual, vital and informative! I really need to rehome or release my Dussumieri soon.


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