Freshwater fish are found in many different types of habitats in nature. Some species are found in cold, fast moving rapids while others may live in warm, nearly stagnant water. Sometimes this can occur even among the same species. Some species of marine fish are known to swim into coastal estuaries to spawn and as the offspring mature, although born in freshwater, they gradually move into the ocean. Some Neo-tropical cichlids venture out into the sea to visit coastal islands to feed and return to their freshwater habitats at the end of the day.
In short, most freshwater fish must be able to adapt to changes in temperature, changes in pH, and/or dissolved solids or organics in their water because seasonal temperature and rainfall (or lack of rainfall) or any number of other factors may alter chemistry in the rivers and streams in which they have evolved.
The important thing to remember is that, although there are exceptions, the vast majority of tropical freshwater fish commonly found in the hobby are produced in captivity (either on fish farms or in breeding warehouses) and will thrive in the following water parameters.
Temperature: 77 to 82 degrees F
Alkalinity (KH): 80 to 180 parts per million (ppm)
Nitrite: ZERO; nitrite is toxic at a pH of 7.0 or lower.
Nitrate: as low as possible.
Salinity: as a general rule, 1.003 ppt is adequate. A few freshwater species (Discus and Danios most notably) are not salt-tolerant. A planted aquarium should not have salt added to the water as most plants are intolerant of salt.
The above water parameters are the ideal conditions at which general community freshwater fish should be maintained. Tetras, Barbs, Danios, Rainbow fish, many Livebearers, Catfish and Plecos, Loaches and Botias, Gouramis, peaceful Neo-tropical cichlids (Angelfish, Severums, Eartheaters, etc) and the many dwarf riverine cichlids (found in both hemispheres) make up the majority of tropical fish commonly found in the hobby and will all live together peacefully as long as they are provided with enough room and food. All of these fish will enjoy an aquarium that includes live plants, although some species will dig into the substrate and others may nibble on the leaves and roots.
Brackish water fish (puffers, dragon gobies, some eels, etc) should be kept at a pH between 7.5 to 8.0 with a KH of 120 to 180 ppm. Salinity should measure 1.005 to 1.010. The temperature should be between 75 and 79 degrees F.
Discus are a type of South American cichlid. They must be kept very warm (87 to 90 degrees F) and will not tolerate even moderate concentrations of dissolved organics (i.e. nitrates) for long. 50% water changes at least twice a week are necessary to keep them healthy and maintain them in an aquarium.
The Rift Lake cichlids of South Africa commonly present in the hobby originate from the lakes Malawi, Tanganyika, and Victoria. The vast majority of the specimens found in the trade are bred in captivity. Rift Lake cichlids are best kept in groups in aquariums of 75 gallons or more with plenty of rockwork and caves. They should be kept in alkaline water with a pH of 7.5 to 9.0 and a salinity of 1.005. The temperature should be 75 to 79 degrees F.
There are several species of fish that remain popular that grow too large and/or are too aggressive for the average home aquarium. Arowanas, Pacu, Piranha, Gar-pikes, giant growing Catfish, and certain species of Neo-tropical cichlids (festae, umbriferum, dovii, peacock bass, etc) and Tanganyikan cichlids (frontosa, b. microlepis) will either grow too large to maintain or are simply too territorial to be kept in all but the largest (125 gallons at minimum; preferably larger) home aquariums. All of these fish are cute when they are babies, but unless the hobbyist is willing to provide adequate space for them as they grow, these fish should not be chosen for the home aquarium.
FRESHWATER FISH FAMILIES
These include the Bettas and Gouramis. Almost all of these fish are good for a general community tank and will thrive in the water parameters listed above.
This group ranges in size from small peaceful Corydoras to giant growing Mekong and European Wels catfish. This group also includes the popular Plecos and Panaques. Some catfish are nick-named “sharks” (i.e. Black Fin sharks, Iredescent sharks). As a general rule the small species are great general community fish. As the larger species mature, they will usually eat their tankmates if they can.
This group includes the Tetras, which are a generally hardy, schooling fish. They are happiest in a large shoal of their own kind. Most species of Tetras do not grow large and can be kept in the general community.
Pacu, Piranha, Hydrolicus, and the Tiger Pikes are also in this family. Because of their size at maturity and/or predatory nature, these species are not a good choice for most home aquariums.
These include Angelfish, Discus, Severums, Oscars, as well as the Jack Dempsey, Green Terror, Jaguar, Red Devil, Red Terror, Umbee, Wolf and Midas cichlids from the new world and the Rift Lake cichlids from South Africa. With the exception of small growing river species (which are found in both hemispheres), these fish are not considered general community and grow large and aggressive. All cichlids are aggressive to some degree when spawning; some species will annihilate all tankmates to protect their spawn.
Water parameters vary with the species.
There are a few hybrid cichlids such as the so called “Blood Parrots” and Flower Horn cichlids that are popular as well. These hybrids should not be kept in a community unless the tank is over 100 gallons.
This is another large and varied family of fish that includes Barbs, Danios, Loaches and Botias, so called “sharks“ (Rainbow, Red Tail, Black, Bala, etc.) and Rainbow fish. Almost all of these fish will be happiest in a large school of their own kind. Generally, water parameters are the same as mentioned above. The smaller species are good general community fish but some Danios and Rainbow fish will be sensitive to even small amounts of salt in the water.
Koi and Goldfish are also in this family. Koi and Goldfish grow large and like cooler water so they aren’t a good choice for a community tank.
This group includes Mollies, Platties, Guppies, and Swordtails. They are best kept in tanks with other livebearers; however they may also do well in a planted aquarium or in a peaceful brackish habitat. Mollies will actually thrive in saltwater.
The text above is intended to be a general guideline; always research the specific requirements of any species of fish you want to keep or breed in your aquarium.