Fish — behavior & socialization (companionship, play, bonding, preventing boredom/behavior problems)
Source: first-hand owner-practical care page, saved to source_file; every claim below is a C1 byte-substring of it.
Claims
As with other fish, different species have different behaviours.Others aren't very social and will fight with either their own kind or other species.For example, some are 'shoaling fish' who feel more secure when kept as a group.Avoid overcrowding The number of fish you can keep in an aquarium depends not only on the water quality but also on the behavioural needs of the fish.Other tropical fish aren't very social and will fight with either their own kind (such as male betta fish) or other species kept in the same tank.Keeping too many fish in the tank means they'll have to compete for food and they may become stressed.This means that you should keep them in groups of six or more, otherwise they may feel stressed.
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Source: pdf-raw/companion-care/exotic_rspca_fish.txt · https://www.rspca.org.uk/adviceandwelfare/pets/fish