Choosing fish is one of the most exciting and hardest things a new aquarist has to do. Many aquarists just go to the fish store and buy the fish they think looks good and come home with an Oscar, an angelfish, two guppies, and a goldfish. This is exactly what you shouldn’t do!
You should start by learning more about the fish you are considering getting. It is better to go to the fish store without buying something and then reading about the species you are considering getting in a book or on the internet. If the person described above had done that he would have noticed that those fish species really aren’t suitable to be kept together. The goldfish prefers colder water than the other species and the Oscar grows large enough to eat the guppy, too large for most beginner aquariums. Not to mention the fact that he might beat the other fishes to death. Another reason to do your own research is that the personnel in the fish store don’t always tell you the truth about which fishes that are suitable to be kept together. Many do but some just want to sell you the fish regardless.
So how do you choose species that are suitable to be kept together? Doing your homework and reading about the species you are considering getting like suggest above is important but you should start by deciding what type of aquarium you want. Do you want a lot of small colorful fish or is the behavior more important than the colors? Do you want to keep peaceful fish or perhaps aggressive predatory fish? Once you have answered these questions and others like them, and only then, can you start considering which species that would suit your intended setup by reading about different species and visiting local fish stores to see what they have to offer? Just visiting the fish store without reading about the species can as shown make you end up with species that are truly unsuitable to be kept in the same tank, but only reading without investigating what is available around you can be just as futile as you can make grand plans that you can’t realize because the fish simple isn’t available in fish stores in your area.
There are a long row of different factors that you should consider when choosing fish. The first group of factors you have to consider revolve around the water. You should only choose fish that thrives in the same type of water. This means water with the same pH, temperature, hardness, and salinity. The second group of factors to observe is the environment/ biotope the fish prefers. Some fish prefers a rocky environment without plants while other requires plants to feel at home and you should of course only keep fish that feel at home in the same type of environment together. It is however sometimes possible to offer two or more types of environments in the same aquarium and you can in that case mix fish from different environments. The third group of factors is those who revolve around the fish themselves such as the size and temperament of the fish. You should only mix fish with a similar temperament together in an aquarium. You can mix fish of different max sizes in the same aquarium but you should avoid mixing species with too large size difference in the same tank.
If you do your homework and consider the factors above you should be able to create a good community aquarium with fish that are suitable to be kept together.