Estimating the Need for Swimming Space for Aquarium Fishes

Jay Hemdal

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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 www.fishbase.org 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, and areas filled with corals and rock must not be measured. 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.




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 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 eye. 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 www.FishBase.com 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
AnglerfishAnthiasDanios
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
Tetras
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.



References



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

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.
 

Tyler Bullock

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You just green lit yellow tangs for a 4 foot tank. I feel like the tang police are going to need some therapy.

20cm=7.87"x0.8=6.3"
48+18=66"
This was the first thing I checked when I saw this. Quite interesting to say the least, and the math is for a max length fish. I can't wait for the tang police to arrive. They might loose their minds.
 

MONTANTK

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You just green lit yellow tangs for a 4 foot tank. I feel like the tang police are going to need some therapy.

20cm=7.87"x0.8=6.3"
48+18=66"
I don’t think you’re factoring in the addition of rock which lessens the open swimming space. So in reality it’s probably more like 48+8 at best. In a 120 though I’d imagine that ratio would support adding a yellow tang
 

Mikedawg

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I was wondering how to mathematically prove to my wife that I need a larger tank! Love the general guidance here, much better then the tank volume numbers seen when purchasing fish.
Here's what I used on my wife, where dz = need for a larger tank. Fortunately, she was an English major, lol. Good luck

1617026626526.png
 
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Jay Hemdal

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You just green lit yellow tangs for a 4 foot tank. I feel like the tang police are going to need some therapy.

20cm=7.87"x0.8=6.3"
48+18=66"
Don't forget - that has to be open space. I see no issue holding a yellow tang in an open tank of that size, but with reef structure, that could very well end up needing to be a six foot tank.

Jay
 

Tyler Bullock

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This thread is for the general discussion of the Article Estimating the Need for Swimming Space for Aquarium Fishes. Please add to the discussion here.
Jay, I do have a question. So I do have a 90 gallon, 48 x 24 x 18. I would say the top 6-8 inches of my tank has zero rock or coral. Mid tank I have about 4-6 inches of rock width. Bottom of the tank I would say some areas have 12 inches, some areas have maybe 6 inches. Since we aren't calculating the height of the tank. What level should I do my calculations?
 

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Jay, I do have a question. So I do have a 90 gallon, 48 x 24 x 18. I would say the top 6-8 inches of my tank has zero rock or coral. Mid tank I have about 4-6 inches of rock width. Bottom of the tank I would say some areas have 12 inches, some areas have maybe 6 inches. Since we aren't calculating the height of the tank. What level should I do my calculations?

I bet we will find lots of gotchas like this.. In my big tank only about 2/3's has rock and then a large open section without any rock at all. I kinda.. took the open section for my calculations but really thats probably a misrepresentation.
 
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Jay Hemdal

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Jay, I do have a question. So I do have a 90 gallon, 48 x 24 x 18. I would say the top 6-8 inches of my tank has zero rock or coral. Mid tank I have about 4-6 inches of rock width. Bottom of the tank I would say some areas have 12 inches, some areas have maybe 6 inches. Since we aren't calculating the height of the tank. What level should I do my calculations?
Some assumptions need to be made - one of them is that the open area is at least 12" deep. If it isn't that deep, don't consider that space in the calculation. In other words, if the rock goes to less than 12" from the surface, don't count that space.

Jay
 

Seabiscuit

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I liked your article. I recently upgraded my tank from 29 gallons to 55 gallons. My small Yellow Watchman Goby was so much happier. He tripled his territory in just the first two days. Most recommendations for his tank size I have seen were 30 gallons minimum. It was interesting seeing the change in his behavior.
 

Tyler Bullock

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Some assumptions need to be made - one of them is that the open area is at least 12" deep. If it isn't that deep, don't consider that space in the calculation. In other words, if the rock goes to less than 12" from the surface, don't count that space.

Jay
Thank you for the clarification, this certainly does help when deciding fish for an aquarium!
 

iMi

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I cranked the numbers and looks like a Tang in my 20 gallon nano is still out of the question. LOL

In all seriousness, great info. I enjoyed the article. Thank you for sharing.
 

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Some assumptions need to be made - one of them is that the open area is at least 12" deep. If it isn't that deep, don't consider that space in the calculation. In other words, if the rock goes to less than 12" from the surface, don't count that space.

Jay

I’m trying to gauge how my néw tank would fit this calculator.
The setup is a 36” long peninsula tank that is 20” wide. The rockwork is in a strip down the middle so there are basically 2 - 36” open area runways on both sides. The scape in the front half has about 12” of open space above it and the back half has about 8-10” of space above it.
 

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Jay Hemdal

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I’m trying to gauge how my néw tank would fit this calculator.
The setup is a 36” long peninsula tank that is 20” wide. The rockwork is in a strip down the middle so there are basically 2 - 36” open area runways on both sides. The scape in the front half has about 12” of open space above it and the back half has about 8-10” of space above it.
I'm not sure it can be used easily on your tank. The best I can come up with is to add up the two linear sections, plus their width, and count it as one open area. There just are some configurations that just don't work out - cylinder tanks for example.

Jay
 

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Jay, I do have a question. So I do have a 90 gallon, 48 x 24 x 18. I would say the top 6-8 inches of my tank has zero rock or coral. Mid tank I have about 4-6 inches of rock width. Bottom of the tank I would say some areas have 12 inches, some areas have maybe 6 inches. Since we aren't calculating the height of the tank. What level should I do my calculations?
In freshwater you can have bottom, middle, top dwelling fish so there’s a more economical use of space. I have an African butterfly fish who lives on the surface, guppies who dwell in the middle, and a loach or freshwater goby who lives on the bottom. All in 12” of water.

In Saltwater, that’s less the case. I have a goby who sifts the sand and one who lives in the sand with his shrimp. I have a dragonet who hovers…but most of my fish occupy the middle of my tank (foxface, tangs, damsel, etc). So depth is important, but nowhere near as much as length on most home aquaria. This is the issue with 120g tall aquariums and people rushing out to buy fish based on the minimum tank size of 125g.

If you have a 4-6’ deep tank that might be a different story but I recently visited a tank that was 36” tall and the fish (tangs, anthias, etc,) never left the bottom 18” of the tank.
 

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