Use this section as the first step in planning your “Aquarium Goal.”
A SPECIAL NOTE ON AQUARIUM SIZE RECOMMENDATIONS: While the gallon size is important, the dimensions are of equal or greater importance. Generally, a tank of rectangular shape is preferred over a tall tank. A 30 gallon tank, for example, comes in a hexagon shape that is about 20″ wide and 25″ tall. It looks great but you can keep at least twice as many fish in a “30 Long” which is 36″ x 12″ x 17″. The long tank has more surface area for oxygen/CO2 exchange and more lateral territory for fish.
Ornamental freshwater fish come from all over the world, and in all sizes and temperaments. If they have greatly different needs for temperature or water pH, or if they have aggressive temperaments, they may have to be kept in groups of fish with similar needs and dispositions.
DISCLAIMER: The following includes general compatibility guidelines based on personal experience. Fish may be “individuals” in behavior and different aquarium setups may foster different behaviors. There are no absolutes, and your experience with regards to fish temperament may vary.
The following are some generalized groups that should be compatible:
Peaceful, small (2-3 inches), community fish – soft water
This group includes most members of the tetra family, rasboras, rainbow fish, Dwarf Gourami’s, Cory catfish. These fish generally get along together, and do best at a pH range of less than 7.6.
These fish may be easily kept in aquariums from 10 gallons on up, at an ultimate capacity of ~1 inch of fish per gallon.
Peaceful, small (2-3 inches), community fish – hard water
This group is comprised primarily of “livebearer” fish, such as guppies, swordtails, platys, and mollies. They are happiest at pH levels exceeding 7.6 and the mollies, especially, due best in partly “salty” water.
The same aquarium recommendations as in the previous group.
Semi aggressive, small to medium fish – soft water
This group includes barbs (some of which may be fin-nippers), standard size (i.e. not “dwarf”) Gourami’s, rainbow and red-tail sharks, giant danios, large tetras such as Buenos Aires tetras. Some larger fish such as severums, blood parrots, and silver dollars may also fall into this group.
Small specimens may be kept in small (e.g. 10 gallon) tanks, but 20 to 30 gallon or larger size is preferable. pH preferred is less than 7.6.
Aggressive fish – soft water
These fish are generally large (up to 12+ inches or much more) fish mostly from Central and South America, or Asia. The includes members of the cichlid family such as Oscars, Jack Dempsey, Red Evil, Green Terror, Texas cichlid, as well as the Clown Knife and Pacu (you shouldn’t even buy a Pacu unless you can accommodate a 3 ft adult fish that lives for many years. Most stores don’t want them, or other members of this group back when they are large) Do not plan to keep members of this group in small aquariums except perhaps when they are juveniles. Think BIG as in a 48″ long tank such as 55 gallons plus. This group is best kept at a pH of less than 7.6.
Aggressive fish – hard water
This group is composed mainly of African Cichlids (those that originate in the wild in Lakes Malawi, Tanganyika and Victoria, in central Africa). A colorful group with many species, these fish are best kept among other African Cichlids, from a compatibility and water quality standpoint. They are disease resistant and tolerant of less than ideal water quality. They can be very territorial, however, and don’t like to be crowded. They are best kept in aquariums of 55 gallons (48″L) of larger, with lots of “structure” in which to establish personal territory (limestone rock is excellent for this). They do best at a high pH, above 7.4 with 7.8-8.3 a better level.
A very colorful group of fish that are not always “gold” in color. These distant members of the carp family have been bred and crossbred for centuries in the Far East, where many manmade “styles” have been developed. Colors vary to include white, black, red and bronze. Varieties include fish with spectacular head growths (Orandas), unusual eyes (bubble eye, celestial eye), fancy skin (Pearlscale), and many more.