Fish of the same location or temperaments. In cichlids keeping them grouped by specific locations (i.e. South American cichlids, New American cichlids, Lake Victoria Cichlids, Lake Tanganyika, Lake Malawi, Madagascar, etc.) helps keep deaths down, though it is possible to mix in some circumstances. There are also what is known as community fish. These are your generally more peaceful types such as mollies, guppies, sword tails, tetras, veritas, etc. And remember, just because a type of fish is suppose to act a certain way, does not mean all of them will.
The best thing to do is ask the shopkeeper in the fish shop and find out first but goldfish go together with no problems if you are lucky because some goldfish can't live together. http://en.wikipedia.org/wiki/Goldfish
gold fish
mud fish
gold fish
the swimming kind
Yes.
The kind of fish that live in the Atlantic Ocean... :))))
starfish.WAIT they're not fish
sea fish.
fish. :)
The kind that swim.
all fish lives in water