Fortunately, for those who have space or budgetary limitations, several fish species, including tetras, killifish, small cichlids and several others, can live comfortably in small aquariums. Because each inch of fish length requires about 1 gallon of water, most 10-gallon aquariums are only suitable for a few individuals. While relatively few saltwater enthusiasts utilize such small tanks, a few species are suitable for small marine tanks as well.
Thanks to their small size -- specimens rarely reach 1 inch in length -- and their relatively inactive lifestyles, neon tetras (Paracheirodon innesi) make excellent fish for small aquariums. Neons also thrive at water temperatures of 68 to 78 degrees Fahrenheit, which means that you may not require a heater to keep the tank warm enough. On the other hand, if you plan to heat the aquarium, Cardinal tetras (Paracheirodon axelrodi) remain small, yet prefer water temperatures between 74 and 80 degrees. Cardinal tetras reach slightly larger lengths than neons do, so you cannot keep as many of them in a small tank.
Shell-dwelling cichlids, such as Neolamprologus brevis often thrive in small aquariums. While the males may approach 2½ inches in length, the females remain less than 1½ inches long. These interesting inhabitants will take up residence in small shells; sometimes a pair will share a shell, at other times each individual will inhabit separate shells. Use caution when introducing additional fish into the aquarium, as the small cichlids will defend a small area around their shells, which may lead to infighting.
Least killifish (Heterandria formosa) are delightful and diminutive fish well-suited for life in small aquariums. These fish do not like strong water currents, so use a filter that only produces a gentle water flow in the tank. Least killifish prefer heavily planted tanks, which give them innumerable places to hide. These live bearing fish often breed readily in the aquarium, even without any special effort on the part of the hobbyist.
Although relatively few aquarists maintain small saltwater tanks, it is possible to maintain such a habitat, as long as you avoid overcrowding your fish. Additionally, you can add some live rock and a few invertebrates to the tank to create your own miniature ocean ecosystem. Some fish may even form symbiotic relationships with shrimp or anemones. A few of the best species for 10-gallon marine tanks include:
Blue neon gobies (_Elactinus oceanops)_ Orange-spotted gobies (Amblyeleotris guttata) Clown gobies (_Gobiodon atrangulatus)_ Firefish (Nemateleotris magnifica) Longspine cardinalfish (_Zoramia leptacantha)_ Yellow stripe clingfish (Diademichthys lineatus)