There are many sources of information available that clearly describe the character of various fish species, such as distinguising which are territorial versus those that are a more peaceful schooling species.
The overall size and mass of the fish is just as important as its length as is the combined lengths of smaller fish. Knowing if the fish is predominantly a herbivore, a carnivore, or an omnivore as well as having an idea of how it behaves when spawning is all-important information when deciding on which fish to choose. For example, a ten-inch Oscar cannot be kept in 10 gallons of water, but 10 adult neon tetras certainly can be. Territorial cichlids need room to establish territory, and if you don’t provide it, there is going to be conflict and damage to the fish.
In contrast, the behaviour of algae-grazing, mouth-brooding mbuna cichlids from Lake Malawi differs from most substrate-spawning cichlids. The former do better in crowded groups because it tends to disperse the aggression that males dish out in defending their now smaller territory.
It is also a good idea to understand the behaviour and activity level of fish you plan to keep. For instance, are they fast-swimming and aggressive feeders, or are they the shy and more retiring species?
You also need to consider the ‘one inch of fish per gallon’ rule as it applies to the adult size, or max growth of the fish and not the size at the time of purchase (if they are still young.)
Remember, there are many exceptions to these rules, and you need to do some research on the species that interest you, however, these general categories are good examples to help you make informed decisions when deciding on the number of fish to stock your aquarium.
SMALL, PEACEFUL SCHOOLING SPECIES: 1-2 INCHES
For 1-2 inches of adult, small-sized species per gallon, good examples include various species of smaller, dwarf species of tetras, guppies, barbs, gouramis, and rasboras.
For 1 inch of adult, medium-sized species per gallon, good examples include most livebearers, medium-sized barbs, mid to larger-sized gouramis, and danios.
MEDIUM-SIZED CENTRAL AMERICAN & SOUTH AMERICAN CICHLIDS: 4-6 INCHES
These fish are territorial. When kept in small groups, for example 6 to 8 medium-sized cichlids i.e. Rainbow Cichlids, Firemouths or Severums, the minimum aquarium size would need to be 55 gallons.
LARGER-SIZED CENTRAL AMERICAN & SOUTH AMERICAN CICHLIDS: 6-12 INCHES
These species are also territorial and are for the most part determined predators. Minimum tank size for a few adults i.e. Oscars, Jaguar Cichlids, Jack Dempseys or Red Devils, would be 100 gallons.
Discus, on the other hand (a cichlid that could be considered a larger fish), is a less aggressive species that schools together in nature. Discus would do well as an adult group of 6 in a 50-gallon aquarium. A dozen adults could easily be housed in a 100-gallon aquarium.
In summary, our recommendaiton is use this general approach when stocking, and understand and research the fish that interest you, but remember one of the most important basic needs is providing an appropriately-sized aquarium.
TOP TEN AQUARIUM FISH FOR BEGINNERS
Fluval’s aquatic experts have weighed in and created their Top 10 list of fish that are best suited for beginners just entering the hobby.