The size of the aquarium is also important in terms of how fish behave. Schooling fish need to be kept in groups of at least five or six specimens, and that it turn requires a certain amount of aquarium volume and swimming space. When kept in insufficient numbers, barbs, danios and tetras become frustrated and often turn aggressive or nippy. Territorial fish need to be able to claim a certain patch of ground, and if there isn’t enough space in the tank, fighting or bullying can occur. Livebearers pose a particular set of problems because of the way males fight with each other while also tending to bully the females. It is important that there is enough space for the male and female livebearers to spread out, and if necessary find hiding places where they can rest or give birth safely.
For all practical purposes, the minimum “safe” aquarium size is 20 US gallons (75 litres). Such a tank will be big enough to accommodate a reasonable selection of small aquarium fish without being particularly large or expensive. More ambitious aquarists interested in big or territorial species such as cichlids should consider larger systems though, with tanks up to 55 US gallons (210 litres) in size providing a good balance between size and expense.
Why keep a small aquarium at all?
Small aquaria can be fun, and obviously they’re easier to fit into a crowded apartment or dorm room. But let’s be clear about one reason never to keep a small aquarium: to save money.
You won’t save money setting up a small aquarium. The price differential between a 10-gallon tank and a 20-gallon tank is trivially small, and the key pieces of equipment such as heaters and filters will likely be the same anyway, so there’s no savings to be had there. Decorating the two different tanks in terms of gravel, bogwood and plants will be pretty much the same as well.
Running a small aquarium won’t save you any money either. Because a small aquarium is more prone to problems, you’ll likely (almost certainly) have to deal with things like finrot, fungus and whitespot on a regular basis. Even after spending money on the medications, if conditions are fundamentally poor or unstable, the chances of the fish dying are much higher than in a bigger aquarium. Dead fish will need to be replaced, and that costs money. Worst case scenario, all your fish die and all your plants rot, so you give up and abandon fishkeeping altogether. However seemingly inexpensive your fish tank might have been, it’s a total waste of money if it isn’t used.
So I say again, if you’re new to the hobby or want a simple, easy to maintain aquarium, always choose a tank at least 20 gallons in size. In the long term, this will be the economical as well as the easier option
5-gallon (19 litre) tanks
Stocking tanks under 10-gallons in size is difficult because relatively few fish can be maintained in such small environments permanently. Popular fish like neons, guppies and Corydoras simply aren’t viable in 5-10 gallon tanks. Your options are really limited to just two things: bettas and freshwater invertebrates.
Bettas (often called Siamese fighting fish) are domesticated varieties of the species Betta splendens. The males have been bred to have exceptionally long, colourful fins. They find it difficult to swim against strong currents, and because of this tend to be rather lethargic animals. While you can keep them in larger tanks if you want, male bettas do perfectly well in tanks as small as 5 gallons in size.
It isn’t a good idea to keep them in small containers though. Very small tanks (or bowls) less than 5 gallons in size are difficult to heat and filter. Bettas are not coldwater fish; they must be kept at around 26-28 degrees C (79-82 degrees F) all day long. Putting an unheated betta bowl below a light bulb for a few hours isn’t an alternative! Similarly, you need to consider filtration. An air-powered sponge filter works great in a 5-10 gallon aquarium and will do a good job of removing ammonia. Bettas exposed to poor quality water invariably become sick and die prematurely, often from completely avoidable problems such as Finrot.
People who breed bettas typically keep the males in bowls, even jam jars, but what casual hobbyists don’t realise is that these bowls are kept in heated fish rooms, and the water in the bowls is changed at least once a day. That’s an insane amount of work for someone who just wants a pet fish: if you want a betta, invest in a 5-10 gallon tank, buy the heater, install a filter, and enjoy your fish doing nothing more strenuous than replacing 25% of the water weekly and periodically cleaning the filter as required.
The freshwater invertebrates most often kept in miniature aquaria are the small algae-eating shrimps. Cherry shrimps, bumblebee shrimps, and crystal red shrimps are a few examples of the pretty little shrimps available to the aquarist. Most shrimps are gregarious, but some will also breed readily if properly maintained. If you look after your shrimps, it is entirely possible you’ll soon have quite a sizeable population of the things in your aquarium! Because shrimps are brightly coloured and very active, they’re fun animals to watch. Feel free to mix different varieties: they all seem to get along nicely. Different species are red, orange, blue and all sorts of other colours, so there’s plenty of scope for creating a busy little ‘freshwater nano reef tank’.
The size of the aquarium is also important in terms of how fish behave. Schooling fish need to be kept in groups of at least five or six specimens, and that it turn requires a certain amount of aquarium volume and swimming space. When kept in insufficient numbers, barbs, danios and tetras become frustrated and often turn aggressive or nippy. Territorial fish need to be able to claim a certain patch of ground, and if there isn’t enough space in the tank, fighting or bullying can occur. Livebearers pose a particular set of problems because of the way males fight with each other while also tending to bully the females. It is important that there is enough space for the male and female livebearers to spread out, and if necessary find hiding places where they can rest or give birth safely. For all practical purposes, the minimum “safe” aquarium size is 20 US gallons (75 litres). Such a tank will be big enough to accommodate a reasonable selection of small aquarium fish without being particularly large or expensive. More ambitious aquarists interested in big or territorial species such as cichlids should consider larger systems though, with tanks up to 55 US gallons (210 litres) in size providing a good balance between size and expense.  Why keep a small aquarium at all? Small aquaria can be fun, and obviously they’re easier to fit into a crowded apartment or dorm room. But let’s be clear about one reason never to keep a small aquarium: to save money. You won’t save money setting up a small aquarium. The price differential between a 10-gallon tank and a 20-gallon tank is trivially small, and the key pieces of equipment such as heaters and filters will likely be the same anyway, so there’s no savings to be had there. Decorating the two different tanks in terms of gravel, bogwood and plants will be pretty much the same as well. Running a small aquarium won’t save you any money either. Because a small aquarium is more prone to problems, you’ll likely (almost certainly) have to deal with things like finrot, fungus and whitespot on a regular basis. Even after spending money on the medications, if conditions are fundamentally poor or unstable, the chances of the fish dying are much higher than in a bigger aquarium. Dead fish will need to be replaced, and that costs money. Worst case scenario, all your fish die and all your plants rot, so you give up and abandon fishkeeping altogether. However seemingly inexpensive your fish tank might have been, it’s a total waste of money if it isn’t used. So I say again, if you’re new to the hobby or want a simple, easy to maintain aquarium, always choose a tank at least 20 gallons in size. In the long term, this will be the economical as well as the easier option 5-gallon (19 litre) tanks Stocking tanks under 10-gallons in size is difficult because relatively few fish can be maintained in such small environments permanently. Popular fish like neons, guppies and Corydoras simply aren’t viable in 5-10 gallon tanks. Your options are really limited to just two things: bettas and freshwater invertebrates. Bettas (often called Siamese fighting fish) are domesticated varieties of the species Betta splendens. The males have been bred to have exceptionally long, colourful fins. They find it difficult to swim against strong currents, and because of this tend to be rather lethargic animals. While you can keep them in larger tanks if you want, male bettas do perfectly well in tanks as small as 5 gallons in size. It isn’t a good idea to keep them in small containers though. Very small tanks (or bowls) less than 5 gallons in size are difficult to heat and filter. Bettas are not coldwater fish; they must be kept at around 26-28 degrees C (79-82 degrees F) all day long. Putting an unheated betta bowl below a light bulb for a few hours isn’t an alternative! Similarly, you need to consider filtration. An air-powered sponge filter works great in a 5-10 gallon aquarium and will do a good job of removing ammonia. Bettas exposed to poor quality water invariably become sick and die prematurely, often from completely avoidable problems such as Finrot. People who breed bettas typically keep the males in bowls, even jam jars, but what casual hobbyists don’t realise is that these bowls are kept in heated fish rooms, and the water in the bowls is changed at least once a day. That’s an insane amount of work for someone who just wants a pet fish: if you want a betta, invest in a 5-10 gallon tank, buy the heater, install a filter, and enjoy your fish doing nothing more strenuous than replacing 25% of the water weekly and periodically cleaning the filter as required. The freshwater invertebrates most often kept in miniature aquaria are the small algae-eating shrimps. Cherry shrimps, bumblebee shrimps, and crystal red shrimps are a few examples of the pretty little shrimps available to the aquarist. Most shrimps are gregarious, but some will also breed readily if properly maintained. If you look after your shrimps, it is entirely possible you’ll soon have quite a sizeable population of the things in your aquarium! Because shrimps are brightly coloured and very active, they’re fun animals to watch. Feel free to mix different varieties: they all seem to get along nicely. Different species are red, orange, blue and all sorts of other colours, so there’s plenty of scope for creating a busy little ‘freshwater nano reef tank’.  Shrimps tend to be quite hardy in some respects, being adaptable to a range of temperatures and water chemistry conditions. Some shrimps come from subtropical and warm temperate parts of the world such as Japan and eastern China, and as such can be kept in unheated aquaria provided your home doesn’t get too cold. Cherry shrimps for example will be quite happy with wintertime temperatures as low as 15 degrees C (59 degrees F). Most shrimps prefer neutral to alkaline conditions, and in general you should avoid keeping them in conditions where the pH drops below 6.5. What all shrimps do demand is good water quality, so filtration is essential. Alongside this it is important to remember shrimps, and indeed invertebrates generally, are killed by copper-based medications such as those used to treat whitespot (ick). Very small tanks tend to lack strong lighting, but you’ll be surprised how well things like Anubias, Java fern and Java moss will do, even at light intensity levels as low as 1 watt per gallon. Java moss in particular works great with shrimps, giving newly hatched shrimps a place to hide for the first few days when they are at their most vulnerable. 10-gallon (38 litre) tanks If I could, I’d make these tanks prescription-only! Ten gallon tanks sound a great idea, but in practise they’re just too small to be useful as general purpose community tanks. They’re great for rearing baby fish or for maintaining breeding pairs of dwarf cichlids. But for your standard issue mixed species community, these tanks are extremely confining.
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- Whiptail catfish, Hemiloricaria spp - Getting to around 4 inches (10 cm) in length, these unusual catfish are relatively inactive and cannot swim well. They prefer to amble about on a sandy substrate, rooting about for food. Easy to maintain on algae wafers, bloodworms and catfish pellets. Keep singly or in groups. Harmless oddballs ideally suited to small tanks.
- Dwarf Corydoras, Corydoras habrosus and Corydoras hastatus - These little catfish are gregarious and should be maintained much like regular Corydoras. Keep in groups of six or more specimens, and feed them with tiny catfish pellets, frozen bloodworms and other small foods. When happy, these catfish are very active and like to swim about in midwater.
- Sparkling gourami, Trichopsis pumila - One of the smallest labyrinth fish and an ideal addition to a densely planted community tank. Peaceful and easy to maintain. Do not combine with larger fish though, and keep away from anything aggressive or nippy.
- Chameleonfish, Badis and Dario spp - Not easy fish to maintain, but brilliantly coloured. Must have live or wet frozen (not freeze dried) foods such as bloodworms and daphnia. Will not eat flake or pellets. Keep in a planted tank with quiet tankmates. Water chemistry not critical.
- Shell dwellers, Neolamprologus spp - There are various dwarf cichlids from Lake Tanganyika that inhabit empty snail shells. These cichlids are territorial but don’t stray far from their home shells, so managing them is quite easy. Neolamprologus brevis is a popular species recommended for beginners. Like all Tanganyikan cichlids it is sensitive to poor water quality and must be kept in hard, alkaline conditions. Colonies are best kept on their own, though some aquarists combine them with small, surface dwelling livebearers such as dwarf mosquitofish.
- Peacock goby, Tateurndina ocellicauda - These small gobies are beautifully marked with yellow, purple and red spots. They are easy to maintain on live and wet frozen foods, but have no interest at all in freeze-dried or flake foods. Males and females are similar, but the males have a stockier profile (especially around the head) and spend much of the time guarding their caves. Will spawn readily in a peaceful aquarium.
- Algae shrimps, Caridina and Neocaridina - Numerous shrimp species are available, but the small algae-eating species from Asia are particularly useful in small tanks. They are active, often colourful, and in some cases breed readily.
- Zebra and pearl danios, Danio rerio and Danio albolineatus - Despite being only a couple of inches (5 cm) in length, these small danios are far too active to be happy in a 10-gallon tank. But in a long 20-gallon aquarium these fish are wonderful, providing plenty of colour and activity. Keep in schools of six or more: in smaller groups, these fish can become nippy. They are also prone to bullying small surface-dwelling fish like White Cloud mountain minnows and fancy guppies, so are best kept with midwater and bottom feeding tankmates. Danios are adaptable fish and generally live for a long time if provided with decent basic care; the main thing to remember is that they are hillstream fish that don’t like temperatures above 77 degrees F (25 degrees C).
- Glowlight danio, Danio choprae - Much like the zebra danio in requirements, these small danios do well in 20-gallon tanks. Increasingly widely sold, these elegant fish are beautifully marked with copper and blue markings on the flanks.
- Tiger barb, Puntius pentazona - These colourful fish have been popular for decades, and are essentially hardy, low-maintenance animals. They are distinctly sociable though, so keep at least six. In smaller groups they tend to become bored and consequently nippy, attacking anything that can’t easily swim away. Angelfish, gouramis and guppies are easy targets. Albino and moss barbs are the same species as the tiger barb, and the ruby barb, Puntius nigrofasciatus, though distinct, requires similar care.
- Harlequin rasbora, Trigonostigma heteromorpha - An excellent schooling fish for the 20-gallon tank, but fussy about water chemistry. Not recommended for aquarists in hardwater areas; generally does best in soft, slightly acidic conditions. Prefers a planted tank with lots of shade.
- Guppies and platies, Poecilia and Xiphophorus spp - These livebearers are sometimes kept in smaller tanks, but the males are aggressive and can become bullies. In larger tanks, they are more easily maintained in groups of one male to every two females. Stock the tank with floating plants if you want to collect any baby fish before the other fish eat them!
Tanks this size are ideal starting points for anyone entering the hobby. In the United States 20-gallon tanks are available in “tall†and “long†varieties. The tall tanks measure roughly 24 inches in length and 17 inches in depth; the long tanks are 30 inches in length and 13 inches in depth. All else being equal, the long tank is better. Long tanks offer more swimming space and have a greater surface area to volume ratio, meaning oxygen diffuses into the tank at a faster rate. You can keep more fish, more happily, in a long tank than a tall tank.
Do tall 20-gallon tanks have any advantages? Not many. They are perhaps a bit easier to decorate with tall plants and rocks, and having a smaller footprint they take up less space on a tabletop or shelf. Greater depth does work better with certain small but tall fish, in particular domesticated angelfish. But beyond that, these tanks are far inferior to long tanks for general fishkeeping and are best avoided by less experienced hobbyists.
Species useful in 10-gallon tanks will do even better in a 20-gallon tank. In the case of things like small tetras, you can keep larger groups. If you have a nicely planted aquarium, consider keeping two dozen neons for example. Otherwise some of the additional species you can sensibly keep in a 20-gallon tank include the following:
- Corydoras - As noted earlier, the dwarf species can be kept in smaller tanks, but most of the popular species such as peppered, bronze and panda Corydoras do best in a 20-gallon tank. Keep them in groups of five or more specimens. Corydoras are generally undemanding, though to avoid keeping them too warm; most species prefer temperatures around 75 degrees F (24 degrees C).
- Dwarf upside-down catfish, Synodontis nigriventris - Gregarious and very hardy catfish; recommended for aquarists interested in oddball catfish. Keep in groups of three or more specimens. Sometimes a fin-nibbler, so best not mixed with fancy guppies, bettas, and other slow-moving fish. Very nocturnal!
- Bristlenose catfish, Ancistrus spp - This is one of the best catfish for the smaller aquarium. Maximum length is about 5 inches (12 cm). Although secretive and essentially nocturnal, this catfish is a good algae-eater and basically easy to keep. Make sure it gets regular feedings of green foods and algae wafers.
- Dwarf rainbowfish, Melanotaenia praecox - A superb schooling fish with a silvery-blue body and pink fins. Maximum length is 2.5 inches (6 cm). As with other rainbowfish, keep in groups of six or more specimens.
- Bolivian ram, Mikrogeophagus altispinosus - Unlike the delicate and disease-prone common ram cichlid Mikrogeophagus ramirezi, the Bolivian ram is hardy and easily maintained in community tanks. Highly recommended as a “first cichlid” for the less experienced hobbyist.
- Kribs, Pelvicachromis spp - The common krib is Pelvicachromis pulcher but there are other species available including Pelvicachromis taeniatus and Pelvicachromis subocellatus all are similar in size and habits. Kribs are territorial, so should be given a cave, hollow ornament or flowerpot that they can defend without disrupting the whole tank. Generally work best with midwater and upper level fish.
- Thick-lipped and banded gouramis, Colisa labiosus and Colisa fasciata - Two small gouramis noted for their bluish body colouration and oblique brick red stripes. Broadly similar to the more widely traded dwarf gourami, but slightly larger at 3-4 inches (7.5-10 cm) in length. Very hardy and easy to maintain, these are ideal gouramis for small aquaria.
- Dwarf gourami, Colisa lalia - Various forms exist, including neon gouramis, sunset gouramis, powder blue gouramis and so on. Widely traded but the farmed stock from Southeast Asia especially is extremely prone to a viral disease that is highly contagious, invariably fatal, but currently untreatable. Best avoided unless you have access to locally bred fish.
- Domesticated angelfish, Pterophyllum hybrid - The standard angelfish of the hobby are hybrids that usually get to no more than 4 inches (10 cm) in length. They are quite tall though, so are amongst those fish particularly well suited to “tall” 20-gallon tanks. They do of course look better in larger tanks, but singletons or pairs can be maintained in 20-gallon tanks without problems. Do note however that angelfish are territorial, and they are also predatory, and will eat very small fish such as neons
- Common community fish unsuitable for small aquaria
Having looked at some fish that make good choices for small tanks, let’s review some of the most widely traded and popular fish that would make bad choices for small tanks. In most cases sheer size is the issue: most anything above a couple of inches (5 cm) in length is likely to find a small aquarium too confining. Large fish make a lot of mess as well, and that will mean that maintaining good water quality will be much more difficult. Ensuring water chemistry stability will be difficult too. The following are fish that will require a tank at least 30 gallons (115 litres) in size, and in many cases significantly more.- Mollies, Poecilia spp - Although popular fish and very colourful, mollies are exceptionally sensitive to poor water quality and pH instability. They invariably do best in spacious, well filtered, slightly brackish conditions. Males also tend to be quite aggressive towards one another, and frequently pester the females as well. An aquarium at least 30 gallons (115 litres) in size is recommended for mollies.
- Swordtail, Xiphophorus hellerii - Swordtails are hyperactive fish adapted to rivers and fast-flowing streams. They need lots of swimming space. Males are notoriously aggressive, and in small aquaria can become bullies. Consider 30 gallons (115 litres) the minimum safe size for swordtails.
- Otos, Otocinclus spp - Despite their tiny size, these dwarf suckermouth catfish are extremely difficult to maintain for any length of time in anything less than ideal conditions. Specifically, they need soft, slightly acidic water that is not too warm; aim for pH 6.5, 10 degrees dH or less, and a temperature no higher than 75 degrees F (24 degrees C). They also need perfect water quality, and do best in tanks with vigourous filtration. Because they feed almost entirely on algae, they should never be put in small or young tanks that lack the requisite healthy fuzz of green algae in the rocks and plants: realistically, this means keeping them in tanks at least three months old and not less than 30 gallons (115 litres) in size. Finally, these are schooling fish, and should only be kept in groups of at least six specimens. Expert aquarists might well be able to maintain a school of Otocinclus in smaller tanks by offering appropriate foods and monitoring water conditions carefully, but causal aquarists would do best to ignore these tiny catfish in favour of hardier species like bristlenose cats and Corydoras.
- Silver dollars, Metynnis and Myleus spp - Various silver dollars are traded, but most get to at least 4 inches (10 cm) in length. They are active, schooling fish that tend to be nervous when kept in insufficient numbers. Best reserved for community tanks 55 gallons (210 litres) or larger.
- Goldfish, Carassius auratus - Goldfish are essentially pond fish, and need as much space as you can give them. If you’re going to keep goldfish indoors, don’t waste your time or money on anything less than a 30 gallon systems (115 litre) system: anything smaller is unlikely to maintain them in good health. Under poor conditions goldfish are prone to finrot, fungal infections and all sorts of other opportunistic infections.
- Non-dwarf Australian Rainbowfish, Melanotaenia spp - Wonderful fish in many ways, these fish are active and get quite large, typically 4-6 inches (10-15 cm) so require much more space than a 20-gallon tank can provide. The smaller species work well in 30 gallon systems; larger species look their best in tanks upwards of 55 gallons (210 litres).
- Silver shark, Balantiocheilos melanopterus - A big, schooling barb-like fish that needs plenty of swimming space. Should be kept in groups of at least five specimens. Maximum size is 14 inches (35 cm) though aquarium specimens tend to be slightly smaller, 8-10 inches (20-25 cm) being typical. Nervous and jumpy, does not do well when “cramped”. Consider a tank 55 gallons (210 litres) in size the absolute minimum, and the bigger the tank, the better.
- Three-spot (blue, gold, opaline) gourami, Trichogaster trichopterus - Although hardy and adaptable, males of this species are distinctly aggressive and frequently become bullies in small tanks. Females may make acceptable additions to tanks as small as 20 gallons (75 litres), but males should be reserved for systems 30 gallons (115 litres) in size or larger.
- Cichlids, family Cichlidae - Apart from some of the dwarf species already mentioned, cichlids tend to be too aggressive and territorial for use in small tanks, even if their body size seems manageable. When spawning, cichlids can be dangerously violent!
- Kissing gourami, Helostoma temminckii - Not particularly aggressive, but large and sensitive to poor environmental conditions. Maximum size is around 12 inches (30 cm) though aquarium specimens are typically only about 8 inches (20 cm) in length. A reasonably good community fish in systems upwards of 55 gallons (210 litres).
- Clown loach, Chromobotia macracanthus - Clown loaches are big, gregarious fish that tend to be fairly boisterous as well. They should be kept in groups, ideally in schools of at least five specimens. Maximum size is around 12 inches (30 cm) though most specimens only get to about one-half to two-thirds that size. Regardless, their sheer size makes them unsuitable for small tanks. They also need good water quality and plenty of water movement, so a big, generously filtered aquarium is essential. Not suitable for anything less than 55 gallons (210 litres), and realistically 75 gallons (280 litres) is a much better bet.
- Common plec, Pterygoplichthys spp - The default algae-eating catfish; hardy, peaceful, widely sold and very inexpensive. But approach this fish with caution! Routinely gets to a length of 12 inches (30 cm) and potentially significantly more. Because of its size and herbivorous nature this fish creates a lot of mess. A big, well-filtered tank is essential. Unsuitable for any aquarium smaller than 55 gallons (210 litres).
- Algae eater or sucking loach, Gyrinocheilus aymonieri - Another widely traded algae-eating fish. Juveniles may seem attractive, but quickly grow into highly territorial, aggressive adults around 10-12 inches (25-30 cm) in length. Not really suited to community tanks at all, and even in rough-and-tumble systems should not be kept in anything less than 55 gallons (210 litres) in size.
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