Stocking a 5 Gallon Tank
From The Aquarium Wiki
Stocking a 5 US Gallon Freshwater Tank
18.9 Litres (5 US G.) tanks are generally considered to be too small for a proper aquarium set up. They are very susceptible to water chemistry swings due to their small water volume. When it comes to stocking such a tiny water volume many things need to be taken into consideration including bioload and activity needs.
Unfortunately these tanks are still used in many small environments such as desktop aquariums, tanks for children and in office spaces. This article will attempt to provide you with stocking ideas and care instructions. When stocking the shape of the tank should be taken into consideration, tall hex tanks or square tanks will be far harder to stock than long rectangular tanks. If you must have a 18.9 Litres (5 US G.), get a rectangular one.
While still small, in order to keep any creatures it will be best kept filtered and heated. It will still need to be put through The Nitrogen Cycle.
Fish suitable for a 5 Gallon Tank
There are very few fish which will thrive in an aquarium so small. The fish listed here should not be considered to be put in together, each would need to be preferably alone and in a species tank. 18.9 Litres (5 US G.) is too small for a community set up.
A single male Betta splendens can live in a 18.9 Litres (5 US G.) as they are not heavy waste producers and are not too bothered about vast amounts of swimming space. While they would appreciate 37.9 Litres (10 US G.) or larger, a 18.9 Litres (5 US G.) will suffice for a single fish. Make sure the tank is well filtered, matured and heated, and provide the fish with hiding places including tall plants (either live or silk, not plastic as plastic will tear their fins). Bettas are not best designed for tall tanks, a rectangular or square 18.9 Litres (5 US G.) is best, although the open top designs are less preferable as Bettas can jump, stick to tanks with fixed lids.
These are tiny livebearing fish, cousins of the Guppy. As they are so small 4-5 males could work in a 18.9 Litres (5 US G.). No more than this though and it is really too small to combine with females and breed. Females will also get larger than males anyway. A 18.9 Litres (5 US G.) could be used as a spawn tank to raise Endler fry which are being bred in a larger tank also. In a planted tank with places to hide and explore 4-5 males should be ok, just be sure to try and get purebred Endlers, Guppy hybrids will tend to be larger than the purebred Endler and be too large for a 18.9 Litres (5 US G.).
Expert Only! This tiny (2.5cm (1")) Puffer's bare minimum tank size is 18.9 Litres (5 US G.). A 18.9 Litres (5 US G.) can house a single fish provided the tank is at least 6 months old, well filtered, heated and the fish is provided with plenty of places to hide and explore. These Puffers are exceptionally sensitive to water quality problems, do not attempt this fish unless you are experienced in maintaining water quality in small volumes of water. The Dwarf Puffer appreciates a planted tank and a secure fixed lid is best. Densely planted is ideal as, as mentioned, these fish need plenty of places to hide and explore as they are exceptionally curious. They will also benefit from snails in their diet to help wear down their ever-growing teeth.
Invertebrates in a 5 Gallon Tank
There are several small invertebrates suitable for a 18.9 Litres (5 US G.), however, they would do best without fish (the shrimp in particular would be a snack to fish like Betta splendens) and very careful attention will need to be paid to water quality as invertebrates are more sensitive than fish. Invertebrate-only tanks are popular alternatives for small tanks and indeed many "nano" specialist shrimp tanks can now be found on sale.
A small group of 10 shrimp will survive and breed in a 18.9 Litres (5 US G.) aquarium, however, it is recommended that if the numbers exceed 10, the extras should be rehomed or upgraded to another tank. If you plan a shrimp tank, it must be noted that these animals are exceptionally sensitive to excess nitrates, so extra attention must be paid to water quality and they do not do well in newly-cycled tanks.
Just like the Cherry Red above, the Crystal Red is about the same size and would thrive in a group of 6-10 in a well planted and matured tank. Just like the Cherries they do not do well in newly-cycled tanks and are sensitive and delicate to nitrates and copper. In a mature planted tank these shrimp may well breed. Extras can be rehomed or moved to another tank.
The Ghost Shrimp are very similar in size to the Crystal Reds and Cherries, however they are more scavengers than algae eaters. They will do well on general shrimp pellets and a fairly densely planted tank. Like the other dwarf shrimp, it will be sensitive to water quality. They will also breed well in tank conditions so extras may need to be rehomed.
One Apple Snail would do well in a well filtered heated 18.9 Litres (5 US G.). A pair could be kept as long as eggs, which are laid above the waterline, were removed as hatchlings would push the bioload over the limit. Be careful to ensure any heaters are out of the snail's grasp as they can burn themselves. They are also likely to eat any live plants.
Unlike the larger more common Apple Snail, the Spixi will not be quite as prolific if kept in a pair as they are also asexual. They will lay their eggs below the waterline though, unlike the common Apple Snail. They prefer a gravel substrate and will spend some part of the day part buried in the substrate.
Other uses for a 5 Gallon
If nothing above tickles your fancy, buy a larger tank. If you are just starting out, a rectangular 56.8-75.7 Litres (15-20 US G.) or larger is ideal. Instead of throwing out the 18.9 Litres (5 US G.), it can be used as quarantine/hospital tank for any new fish or invertebrates for a larger tank, or even a grow out tank for small fry such as Endler's Livebearers or Guppys until they are large enough to re-enter the main tank (sooner rather than later for larger fish fry like Guppys).