When matched with an appropriate lid, 20-gallon aquariums make satisfactory enclosures for a variety of small pets, including several different reptiles, amphibians, invertebrates, fish and mammals.
While all turtles and crocodilians grow much too large for 20-gallon habitats, a variety of lizards and snakes thrive in cages of this size. Leopard geckos (Eublepharis macularius), five-lined skinks (Plestiodon fasciatus) and crested geckos (Rhacodactylus ciliatus) are all great choices for such cages. Snakes that work well in 20-gallon aquariums include rough green snakes (Opheodrys aestivus), red milksnakes (Lampropeltis triangulum syspila) and hognose snakes (Heterodon spp.).
A variety of frogs and salamanders will live well in 20-gallon aquariums. Use a 20-gallon “long” tank for terrestrial and aquatic species, such as fire-bellied toads (Bombina ssp.) or American toads (Bufo americanus); but opt for a 20-gallon “tall” tank for arboreal species, such as red-eyed tree frogs (Agalychnis callidryas).
If you decide to keep aquatic frogs or salamanders, such as African clawed frogs (Xenopus laevis) or fire-bellied newts (Cynops orientalis), you will need to install filtration equipment and a water heater. Be aware that many aquatic species, such as fire-bellied newts, require a small land area in their cage.
Twenty-gallon aquariums make excellent enclosures for invertebrates, and the ample space such tanks offer allow you to decorate elaborately, if you wish. Add some organic soil and live plants to create a tropical habitat, perfect for a pinktoe tarantula (Avicularia avicularia) or an African giant millipede (Archispirostreptus gigas). Alternatively, you could fill the tank with sand, manzanita branches and small succulents to create a desert habitat for a desert hairy scorpion (Hadrurus arizonensis). Be sure to use a mesh aquarium lid when housing any invertebrates.
A variety of fish species can thrive in 20-gallon aquariums, and although it may be challenging to do so, you can even keep saltwater species in a tank of this size. Whether you opt for freshwater or saltwater species, avoid the temptation to overcrowd the tank. While innumerable variables can alter the algebra, a good rule of thumb is to add no more than one inch of fish for every gallon of water, but an even greater ratio of space to fish is preferable.
Some of the most suitable fish for 20-gallon aquariums include:
Twenty-gallon aquariums are suitable for a variety of small rodents, including hamsters, gerbils and mice. You must use a mesh aquarium lid for optimal ventilation, but aquariums are more secure than wire cages. By adding a wire cage to the top of the aquarium, you can offer more room for exploration and exercise. Most common rodent pets thrive at room temperatures, so avoid placing them near heat sources or sunny windows.