Here are some suggestions for groups of fishes that are compatible inhabitants for 5 gallon tanks.  I have only included fish is that are at least relatively easy to keep.

Note: I am assuming a standard 5 gallon long tank. If your 5 gallon tank is tall and narrow, it will support fewer fish as it is surface area that determines the number of fish you can have. There are a variety of rules of thumb people use for determining number of fish. For a properly-filtered tank of standard proportions containing tropical species, a 5 gallon tank should contain no more than 10 inches of fish. I have kept the fish inches here to 8.5 or less because this leaves more room for equipment malfunction such as short power outages and human error.

Cold water fish need more space – no more than one inch of fish per gallon in a standard-sized tank.

The tanks:
5 gallon tank with plants and a pH of between 6 to 7.

Fish Tank One: Amazon Nano Aquarium 1
6 Neon Tetras
1 Otocinclus

Fish Tank Two: Amazon Nano Aquarium 2
6 Black Phantom Tetras

Fish Tank Three: Betta Bachelor Pad
1 male Betta

Fish Tank Four: Lady Betta and Frogs
1 female betta
2 dwarf frogs
1 Otocinclus

5 gallon tank with a pH of 6.5 to 7.5

Fish Tank Five: Subtropical Stream
4-5 White Cloud Mountain Minnows

Fish Tank Six: Mini Catfish Tank
6 Pygmy Corydoras
1 Otocinlus

5 gallon tank with plants and a pH of 7.0  to 7.6

Fish Tank Seven: Where’s the Fish?
8 shrimp of any mix of species that doesn’t exceed one inch, Cherry shrimp is a good beginner species
1 horned nerite snail

Fish Tank Eight: Guppies, Guppies, Guppies
5 Male Guppies
1 horned nerite snail

This is important for the fishkeeping hobby because a surprising number of fishes are still wild-caught from the Amazon, including such popular fish as Cardinal Tetras. This is not the only major recent drought. There was another large drought in 2005 described as a once in a century drought, and repeated droughts of this nature can hardly help but cause damage to the forest and its fish.

Given the predicted effects of climate change on the Amazon I was expecting something like this to happen. This year’s drought is probably partly caused by El Nino, but previous El Ninos didn’t cause this much devastation, and I blame the difference on climate change. See my previous post on the subject. However, I wasn’t expecting it this soon. There is more detail on the 2010 drought in this article by Heiko Bleher in Practical Fishkeeping: The Greatest Amazon River disaster in history.

Given the major problems aquarium fish in the Amazon are facing, you may wish to avoid buying wild-caught Amazonian fishes for the next year or so, unless you intend to breed them and have the skills to do so.

Small Corydoras Species:

C. osteocarus 1.8in
C. tukano 1.8in
C. melini 1.8in
C. breei 1.8in
C. boesemani 1.8in
C. bicolor 1.8in
C. sanchesi 1.6in
C. paragua 1.6in
C. ortegai 1.6in
C. multimaculatus 1.6in
C. gracilis 1.6in
C. lamberti 1.5in
C. paucerna 1.4in
C. habrosus 1.4in
C. cochui 1.4in
C. pygmaeus 1.3in
C. hastatus 1.3in

Sources:

Planet Catfish for species lengths.

Unfortunately, it is also illegally wild caught in various places in Southeast Asia. Captive breeding, poaching, and smuggling are all encouraged by the high prices these fish bring.

Captive bred fish from Singapore usually come with an embedded microchip to prove that they are captive bred and prevent theft. Asian Arawanas are selectively bred for color, especially red or gold variations. There is a tradition in China that this fish, especially if red or golden, brings good luck to its owner. This is one reason for the fish’s popularity and high price.

Because of the prices these fish sell for, farming this species is quite lucrative and has been described as a ‘gold rush’ on the Arowana club website. A very good specimen may be worth tens of thousands of dollars.

Because Asian Arawanas are so expensive, they are sometimes smuggled. This means that if you are looking to buy one, you should check its papers and that it has an identifying microchip. It is all too easy to find stories about Arowanas being confiscated by customs officials, whether in boxes under legal items, or inside a dress being worn at the time. Problems you just don’t get with Neon Tetras…

Sources:

Arowana-care.com

Arowanaclub.com

CITES Convention Text

Fishbase.org

Ng, P., Tan, H. (1997) Freshwater fishes of Southeast Asia: potential for the fish trade and conservation issues. Aquarium Sciences and Conservation. Vol 1: 79-90

UNEP-WCMC. 6 March, 2007. UNEP-WCMC Species Database: CITES-Listed Species

Problems With Betta Bowls

However, the waters from which they come are warm: the average temperature is 24-30C (75-86F), according to Fishbase.org. This is much higher than the temperature in most people’s houses, especially in the winter. While they will survive in an unheated bowl provided the room is decently warm, housing them like this seems questionable. When kept at lower temperatures, Bettas are less active and are more susceptible to disease.

While Bettas do much better than most fish in low-oxygen conditions, they still suffer from build up of fish wastes when kept in small bowls. Such bowls must be cleaned much more frequently than a tank to prevent build-up: 100% water changes multiple times per week. The long fins of male bettas are susceptible to finrot, and dirty water increases the likelihood of problems.

Some of the objects in which Bettas are kept are too small for any fish. In petstores, they are often kept in hand-size bowls, or worse, in plastic drinking cups. I’ve even seen a few in containers narrower than their body was long, not counting the fins. Moral sentiment and the fish’s health aside, this is not a sensible way to sell these fish, as they cannot display properly and their beauty is completely invisible.

Advantages of a Heated Aquarium

Keeping these fish in an adequate-sized and warm tank has advantages to the fishkeeper in addition to feeling good about the way you keep your fish. Healthy fish display more, build bubble nests and check every nook and cranny of the aquarium for missed food. They are much more interesting to watch, and a betta doing an aggressive display in a well-lighted aquarium is a truly stunning sight.

Sources:

Fishbase.org

RandomWiktor. Tiny Mud Puddles and Other Lies: Myths and Misconceptions About Our Beloved Bettas

It has been introduced to Indonesia (Sulawesi), the USA, Hong Kong, Taiwan, China, UK, Papua New Guinea, Guam, Taiwan, Thailand, and possibly the Philippines. This species has been nominated as one of the world’s top 100 invasive species. The method of invasion involves escapees from the pet trade as well as aquaculture. It is illegal to keep this fish in some countries, as well as in many US states.

Very cold water kills Walking Catfish, so if your area goes below freezing in the winter this fish should not become invasive even if some manage to escape. In many areas with warmer water it is often illegal to keep them without a permit. Even if they are legal in a warmer area, it is still very important not to let them escape from their tank.  Because of their ability to walk on land, the possibility of escape is much greater than with most tropical fish.  Make sure their aquarium has a strong and tightly fitting lid so that they cannot push it up and wander off.

Sources:

Global invasive species database. Compiled by Invasive Species Specialist Group (ISSG)

Brogan, Christine. 2003 Introduced Species Summary Project

Goldfish, Carassius auratus: Superb in Large Unheated Aquariums

Goldfish grow.  Common goldfish can grow up to approximately 12 inches in length which is larger than some of the containers the juveniles are kept in.  They also produce a good deal of waste as they grow.   Fancy varieties usually stay smaller than common goldfish, but their fancy fins are inclined to develop finrot in dirty water.  This makes the lack of a filter problematic.  Goldfish are cold water fish so the lack of a heater is perfectly fine.

In short, they are ideal for a large unheated aquarium with filtration but are not suitable for a bowl.

White Cloud Mountain Minnow, Tanichthys albonubes: Good for Small Unheated Aquariums and Large Bowls

White cloud mountain minnows prefer lower temperatures than most tropical fish and for this reason they are often kept in unheated aquaria.   Their natural temperatures are 64-71F.  They are very small fish, staying under two inches in length.  However, they do school so is important to provide enough space for at least a couple of fish, preferably more. Fortunately their small size means that two or three can be kept comfortably in a 2 or 3 gallon aquarium. Of all the fish discussed here, these are probably the best for a large unheated bowl or small unheated aquarium.

Zebra Danio, Brachydanio rerio: Not Ideal

Most danios are small, the zebra danio being 1¾ inches in length, and they are tolerant of a wide range of temperatures.  Their natural temperature range is 64-75F.  However, they are fast swimmers and like to school which makes keeping them in a tiny aquarium questionable.

Watching a school of these fish in a large tank is a completely different experience from watching one or two in a tiny tank.  In a large tank they can get up speed and they seem to fly through the water in a group.  It is beautiful, but it is something you will never see if you put these fish in a betta cube.

Betta, Betta splendens: Ideal for Small Heated Aquariums or Bowls

Bettas stay small, usually growing to approximately 3 inches in length.   The waters from which they come are naturally rather stagnant and they have the ability to take in oxygen from the air.  This greatly increases their resistance to foul water.  They dislike fast filtration.  They are solitary in temperament and swim slowly.  This makes them seem like an ideal candidate for the small unfiltered bowl.

However, the waters from which they come are warm: the average temperature is 24-30C (75-86F), according to Fishbase.org.  While they will survive in an unheated bowl provided the room is decently warm, keeping them in an unheated aquarium seems questionable.

Some of the object in which Bettas are kept are too small for any fish.  They should have room to turn around easily and room to spread those magnificent fins of theirs.  Keeping these fish in an adequate sized and warm tank has advantages to the fishkeeper in addition to feeling good about the way you keep your fish.  Healthy fish display more, build bubble nests and check every nook and cranny of the aquarium for missed food.  They are much more interesting to watch, and a betta doing an aggressive display in a well lit aquarium is a truly stunning sight.

It doesn’t take that much effort to set up a small aquarium with a heater, and this fish will reward you well if you do.  Secondhand equipment can often be bought for less than new equipment and a 2 gallon aquarium is more than enough space for a Betta.

Least Killifish, Heterandria formosa: Ideal for Small Aquariums or Bowls

This is a very small fish native to North America which is related to the guppy.  The Males are extremely small at 3/4 inches, and they prefer normal tropical tank temperatures of 68-78F.  Their small size and tolerance of moderate temperatures makes them ideal for a small unfiltered aquarium, provided it is not subjected to cold temperatures.   The largest problem with these fish is likely to be finding them, since they are not common in aquarium stores.

Invertebrates: Better Than Fish for a Bowl

Freshwater invertebrates may work well in truly tiny tanks if you do not wish to keep fish.  Freshwater shrimp are harassed by many fish species and may do better on their own.  There are also many species of snail, and a shrimp-like creature known as Triops.  Different species have different temperature tolerances, but some of them tolerate cool water very well.  You could also try brine shrimp if you are willing to handle very salty water.

Note

All temperature information is taken from fishbase.org, and are the temperatures at which the fish have been found living in the wild.  Some species can tolerate wider ranges.

Sources:

Butler, Rhett A. 1999-2008. Mongabay.com – San Francisco, CA

Practical fish keeping http://www.practicalfishkeeping.co.uk/pfk/pages/home.php

Mills, D. 1987 Illustrated Guide to Aquarium Fishes. Galaxy Press, England

They will happily eat sinking tablet foods. There are several different types of these which can be found at most pet stores. They also enjoy frozen foods, especially frozen blood worms. Live worm-type foods are eagerly accepted so long as they are small enough for the fish to eat.

Types of Corydoras

There is an astonishing number of Corydoras species. The number of named Corydoras species was 156 in 2006 (Fuller, 2006), with new species being discovered faster than they could be named. These fish originally come from South America, mainly from the great river basins of the Amazon, the Orinoco and the Rio Negro.

The most common Corydoras species in the tropical fish hobby is the Bronze Corydoras, Corydoras aeneus, followed by the Peppered Corydoras, Corydoras paleatus, but there are many are other species available. They range in size from scarcely over an inch in the case of Corydoras hastatus, pygmaeus, and habrosus to approximately 3 inches for Corydoras aeneus and others. Fish that look like Corydoras but are larger are probably in one of the genera Brochis or Schleromystax, both of which are closely related to the genus Corydoras. The common Corydoras are usually between two and three inches in size with Corydoras aeneus being approximately three inches when adult.

With so many species, there are many different color patterns available. Most Corydoras patterns contain contrast in dark and light coloring, or large amounts of gray. Different color patterns will appeal to different people, but the intricate pattern of spots on Corydoras trilineatus makes it one of the most beautiful species.

Corydoras Behaviour

Corydoras school. Almost all species are bottom dwellers and enjoy digging in the gravel. The schools often sit on the bottom or hover and swim just above it. The exceptions are the dwarf species which will often be found schooling in mid-water as well as near the bottom. These smallest corydoras have a slightly different and more streamlined shape when compared to the Bronze Corydoras. The shape of these small Corydoras is probably due to their preference for mid-water swimming where a more streamlined shape is useful.

Corydoras can absorb oxygen from air taken into the gut and for this reason will sometimes dart to the surface for a breath of air and then back down to the bottom. This is normal behavior, but if all your Corydoras are doing this constantly then there may be inadequate dissolved oxygen in your tank.

I have kept Corydoras for many years and I have never seen recognisable aggressive behavior by Corydoras against members of their own species or any other, including towards much smaller individuals that have beaten them to the food, and partially grown fry of other fish species.

Corydoras are egg-depositors and will lay eggs on any available flat surface including the walls of the tank, the tank heater, and tank decorations. Reports on whether they eat their eggs are mixed so you may wish to separate the eggs from the adults.

With their even temperament, myriad species, and comical appearance, Corydoras are much more than just tank scavengers. When given what they need to flourish they are a fascinating and beautiful addition to the tank.

References:

Fuller, I. 2006 Starting with Corydoras. http://www.scotcat.com/articles/article33.htm

Some of these species are sometimes illegally wild-caught and exported from their countries of origin; just because it is on this list is not a recommendation for keeping it.

Enormous Tropical Aquarium Fishes (Over 16 in.)

Red Tail Catfish: Phractocephalus hemioliopterus
Arapaima, Pirarucu: Arapaima gigas (potentially to 16 ft. usually less)
Arowana: Osteoglossum bicirrosum, Osteoglossum vandelli
Black Arowana: Osteoglossum ferreirai
Asian Arowana/Green Arowana: Scleropages formosus
Australian Arowana: Scleropages jardini
African Arowana: Heterotis niloticus
Tiger Shovelnose Catfish: Pseudoplatystoma fasciatum
Shovelnose Catfish: Sorubim lima
Reticulated Pimelodid: Perrunichthys perruno
Plecostamus, Pleco, Suckermouth Catfish: Hypostomus plecostomus
Snow King Plecostomus: Liposarcus anisitsi
Blue-eyed Panaque, Blue-eyed Plecostamus: Panaque suttoni
Sailfin Plecostamus: Pterygoplichthys gibbiceps
Walking Catfish, Albino clarias: Clarias batrachus
Fire Eel: Mastacembelus erythrotaenia
Ornate bichir: Polypterus ornatipinnis
Knife Fish; Featherback: Notopterus chitala
Mozambique Mouthbrooder: Oreochromis mossambicus
Gourami: Osphronemus goramy
Black Shark: Labeo chrysophekadion

Sources:

Butler, Rhett A. 1999-2008. Mongabay.com – San Francisco, CA

Practical fish keeping http://www.practicalfishkeeping.co.uk/pfk/pages/home.php

Mills, D. 1987 Illustrated Guide to Aquarium Fishes. Galaxy Press, England