Another rule of thumb utilizes the ratio of water surface / fish size. Although this may work — to some extent — for ponds and temperate fish species, it can’t be effectively used with tropical species. This rule suggests three square feet of water surface per foot of fish body length. It is easily understood that fish as large as one foot may have a different behaviour in the tank than small fish, and thus, different needs.

A rule based on liters of water per gram of fish mass might be considered far more useful, especially when it comes to large fish (over 20 cm in adult length). However, in this case the problem is the inability to measure the weight of the fish or get information about how much it will weigh when it grows to adulthood. A rough estimation is 500 g of fish mass per 30 cm of fish body length. This is very approximate with too many exceptions which make it practically useless. For example, a slim fish has much less mass than a wide and tall one.

One important aspect of this discussion is that we should always think in terms of adult fish size even if what we have in our tanks now is juveniles or fry (unless we are absolutely sure that the fish will never reach adulthood). Therefore, the rule must rely on what we “see” (physical dimensions of the adult fish itself) and what we know about this particular species (ambush predator, cruise predator, constant swimmer, fish that stays mostly in the bottom, aggressive, territorial etc.). Of course, even this rule will have its exceptions; sometimes it will lead to really large tanks although the fish can make it in smaller quarters. Nevertheless, it will always give you the “order of magnitude” you need to have in mind before buying it. Moreover, in some cases, common sense will dictate a “manual override.” For instance, it is evident that freshwater rays would like to have more surface than height; thus, a necessary adjustment should be made.

et’s see how a “formula” would work if we take some parameters into account and try to implement them in actual situations. The size of the tank is very important, not only in relation to which fish (and how many) can be housed in it, but also for the design of an efficient filtration system.

Tank Length. This is perhaps the most crucial factor in a tank, since it is the dimension which will keep fish as far apart as possible and give them the space to swim and flex their muscles — especially for cruise predators. In our opinion, the tank should have a minimum of 5x adult fish body length (FL) as its long dimension with 20 cm of fish length being the bare minimum. A further 20-50% extra length should be added if the fish is a cruise predator (+30%) or extremely aggressive (+20%) or both (50%); factors of 1.3, 1.2, and 1.5 respectively. If a fish is a poor swimmer then we can get along with just 5x without any additional length despite the aggression factor (0% additional length; factor 1).

We must emphasize the need for first class information about the species you want to keep before you build or buy the tank, and before buying the fish. On the one hand, you should add an extra 10-20% in the L value (factor 1.1 — 1.2) if you want to keep more than one specimen of the same species, while sometimes a second specimen of the same sex can’t be housed in the same tank no matter how generous the dimensions are. It is also wise to recall that, unlike mammals, for instance, fishes have indeterminate growth — there is no absolute maximum size; the fish’s growth slows down eventually, but it will continue to grow as long as it lives, given adequate food. Therefore, a published “maximum size” for a fish species may be routinely exceeded in captivity, and this too should be allowed for in sizing the tank. On the other hand, for really mild-mannered fish kept in a peaceful community tank one can reduce the 5x factor to 4x — but please make absolutely sure that the fish doesn’t become aggressive during spawning or any other phase of its life. It goes without saying that housing a predator with its natural prey in the same tank will not save the latter no matter what the tank volume is. As said at the beginning, common sense is always needed.

Thus (for a single specimen), where L = tank length and FL = expected maximum fish length:

Peaceful fish, peaceful tankmates : L = FL x 4

Aggressive fish, good swimmers : L = FL x 5 x 1.2

Aggressive fish, poor swimmers, ambush predators : L = FL x 5

Aggressive fish, cruise predators : L = FL x 5 x 1.5

Mild temperament, cruise predators : L = FL x 5 x 1.3

Pair of aggressive fish, cruise predators : L = FL x 5 x 1.5 x 1.2

Tank Width. We also feel that the tank should have a minimum of 1.2x adult fish body length as the width. With fish which have a body width over 5 cm, the factor should become 1.3x adult fish length while for fish with body width over 10 cm the factor should be 1.4x. For any additional 10 cm in body width, you should increase the factor by 0.1. Thus, a redtail catfish, with a body width of 30+ cm, should have a factor of 1.6x (since the adult fish has a length of 120 cm, this becomes a 192 cm width). The resulting values take the volume occupied by common aquascaping into account.

Thus (for a single specimen), where W = tank width and FL = expected maximum fish length:

Fish body width <5 cm : W = FL x 1.2

5 cm < Fish body width < 10 cm : W = FL x 1.3

10 cm < Fish body width < 20 cm : W = FL x 1.4

Fish body width = 40 cm : W = FL x 1.7

Tank Height. For height we would recommend a bare minimum of 20 cm and then you have to add 2.5x fish body height (FH) for fish with a body height of less than 5 cm and 3.5x the fish body height for fishes with a body height over 5 cm. For fish that stay at the bottom you may use a factor of 2x body height.

Thus (for a single specimen):

Fish body height < 5 cm : H = 20 cm + (FH x 2.5)

Fish body height > 5 cm : H = 20 cm + (FH x 3.5)

Fish body height < 5 cm; catfish or freshwater ray : H = 20 cm + (FH x 2)

Fish body height > 5 cm; catfish or freshwater ray : H = 20 cm + (FH x 2)

Thus, for fish over 20-25 cm in adult body length the tank size should be calculated by the formula L x W x H as previously described.

s you can see, for a male of the neotropical cichlid Parachromis dovii (6.8 Kg fish; 70 cm adult length) the length should be at least 525 cm (body length 70 cm x 5 x 1.5 since it is aggressive and a good swimmer), the width should be at least 98cm, the height should be at least 90 cm etc.

L= 70 x 5 x 1.5 = 525 cm

W= 98 cm

H= 90 cm

Result = 4,630 liter tank

Pair= 5,093 liter tank

More females or males are not recommended for this species. (Ad Konings in his book Enjoying Cichlids raises the L figure to 700 cm.)

For a male Parachromis managuensis (1.5 Kg fish; 50 cm adult body length), which is equally aggressive but not a good swimmer, the formula gives:

L= 50 x 5 = 250 cm

W= 60 cm

H= 60 cm

Result = 900 liters

For a pair you should add 10% which gives a result of 990 liters.

More females or males are not recommended for this species.

For an Astronotus ocellatus (45.7 cm adult body length; maximum weight 1.58 Kg), which is aggressive but not a cruise predator we will use a +10% for the length:

L=45.7 x 5 x 1.1 = 251 cm

W= 64 cm

H= 90 cm

Result = 1,445 liters

Pair = 1,730 liters

Third individual = 2,000 liters

Fourth individual = 2,300 liters.

For a male of the predatory Malawi cichlid Buccochromis lepturus (adult size almost 40 cm) the calculations are as follows:

L= 40 x 5 x 1.3 = 260 cm

W= 48 cm

H= 45 cm

Result = 561 liters

Second individual = 670 liters.

Third individual (female recommended) = 804 liters.

If more than one species are to be kept together then calculations should be made separately for each species and then choose the largest volume as the correct one, provided that the species are compatible. What you will get is, of course, the right tank for the species demanding the most volume. Then add 5-10% water volume for each additional individual.

e have already mentioned the need of a “manual override” of the above formulas in the case of a species with a specific, odd or unique requirement when kept in captivity. A good example of this is the elephantnose fish, Gnathonemus petersii. When you search the literature you will find out that this species will reach a final length of 35 cm and should be housed in a tank with a length of approximately 80 cm. This kind of tank usually has a capacity of 75 — 80 liters of water. However, this species is a schooling fish and is found in fairly large schools in nature. Keeping less than 6 individuals in a tank will most probably result in increased intraspecific aggression and territorial behaviour. Keeping six or more of them will spread this behaviour equally among them and therefore we will have fewer fights in our tank. Keeping six of those fishes in an 80 liter tank is definitely out of the question. The fish is carnivorous and six of them will present an enormous bioload that will challenge any filtration system.

If we use the methodology we explained earlier in this article we get the following:

35 cm x 4(*) = 140 cm of tank length (*We will consider the species as a peaceful one, rather than a mild temperament).

According now to its body width we must provide the specimen with a tank of 35 cm x 1.2 = 42 cm of tank width.

As far as the height of the tank is concerned, our calculations show that H = 20 + (10 cm x 3.5) = 55 cm. So we’ll need a 323 litre tank to house a single individual.

With the addition of 5 more fish we will have to add 20% per fish which makes a total of 650 liters for six individuals. That’s definitely a long way from those 80 liters we thought would suffice. If we take into account that this species lives in deep waters in the shade of floating plants then we will have to add an extra 50% to the height of our tank to meet this species’ specific needs. This will raise the total water volume to 1,300 liters. Believe it or not, this is the correct type of tank for 6 adult Gnathonemus petersii. Coupled with hiding places, subdued lighting etc., those fish will thrive in such a tank. This is what a real “fish hobbyist” would choose for this fish. In our minds, there are several classifications in the hobby starting with the “tank owner,” the “aquarist,” the “devoted aquarist,” the “fish keeper,” the “fish breeder,” and ending with the “fish hobbyist.” Differences in the wording may mean very little but the first would go for an 80 liter tank while the last might even consider a 1,500 liter tank for the same six fishes.

The differences may not be obvious between a plain tank owner and a fish hobbyist, but there is one — a fundamental one; the plain tank owner only wants a nice looking tank for his or her living room to entertain his or her friends. Something like an expensive piece of furniture, which will create a “nice corner.” In this case, the species kept are just a variety of fish with colours, no matter how they live in their natural biotope. On the other hand, the fish hobbyist will do a lot of research and will try to keep his or her species under the ultimately optimum conditions, will try to breed the species, feed them properly, etc. Breeders will sometimes make some (or a lot of) compromises, due to business demands, so we have put them lower in the “scale” we mentioned above.

Therefore, some crucial factors for estimating the tank size that will keep some species thriving in captivity for many years are an in depth knowledge of their natural environment, their dietary needs, their general and particular behaviour, and some of the special characteristics of these species — if any.

Some of the questions that MUST be answered before selecting any species (and definitely before buying / building any tank) include:

• Is the species nocturnal or diurnal?
• Is it a schooling species or does it live solitarily?
• Does it form pairs, breeding groups, or is it a colony breeder?
• What is the preferable spawning substrate of the species?
• What is its natural prey and how does the species prey upon it?
• Is it a secretive animal or does it swim in the open? …

We could add hundreds of questions like this but these are fine — for a start.

f course, the calculations presented in this article result in the optimum volumes for any kind of fish. With some tricks and clever aquascaping (hiding places, territory separators) we are able to keep them in somewhat smaller quarters, but you have to keep in mind that those values represent their actual needs. Thus, a Buccochromis lepturus needs those 260 cm of tank length and needs them badly. We are sure you will hear of hobbyists keeping this fish in smaller tanks and thinking they are thriving. Take our word for it, they are not — they just can’t tell us that they aren’t. We don’t want to refer to the redtail catfish (Phractocephalus hemiliopterus; adult size 130 cm, >40 Kg) or similarly sized fishes which are kept in 500 liter tanks — or even smaller ones — while the owner hopes that the fish will not outgrow its tank. With a simple calculation, this fish will be seen to need a tank well exceeding 10,000 liters in volume and this is for one fish only. We admit that the fish will feel happy even in a 4000-5000 liter tank but there is no chance to thrive in anything less than that. We once saw an adult pair of them swimming in a 4800 liter tank and it was the bare minimum for them — if not less than that. We simply call keeping this giant in a 500 liter tank a crime and we don’t think that the owner should be called a fish hobbyist. However, part of the guilt is on the petshop owners’ side who continue to market this kind of fishes even though they know that they will never be kept as they should. The difference here is that petshop owners are supposed to care about your money and you, the hobbyist, are supposed to care for the fish. It is common sense that if nobody buys this kind of fishes (except the rare aquarists who have the necessary tank space), petshop owners will not import them any longer and, sooner or later, the fish will happily swim free in the Amazon — or at least in suitable tanks.

We feel that our approach will give a rough estimation for freshwater species. Of course, these formulas could be further refined and specialised; however, this will finally result in one formula for each particular species and make it very difficult to use. Therefore, we will leave them as they are and we hope that common sense will allow the good hobbyist to make the necessary manual adjustments.

Page first posted: 7 September 2003 Web Author: M. K. Oliver, Ph.D. Copyright © 1997-2021 by M. K. Oliver, Ph.D. - ALL RIGHTS RESERVED