Human beings build or buy their homes depending on their financial capabilities and requirements. If they are not happy with their neighbors they can move to another location of their choice. Unfortunately, the aquarium fishes live in whatever space we provide them and we choose the company they keep. Therefore, before you buy any aquarium you must first decide the types of fish you like to have. In this way, you can design a housing system close to their natural habitat. It is also easier to keep the same species of fish together in one aquarium. You can also keep fishes of different species provided they are compatible with one another, have the same nutritional requirements, and can live in similar water conditions in the same community tank. Try not to keep hard water and soft water species together or solely carnivorous (meat-eater) and vegetarian species together. However, most aquarium fishes are omnivorous, eating a combined diet and these species can live with carnivorous and vegetarian species. Some fish (such as Oscar fish) can grow up to 8 inches or more in size and it is unwise to keep them with Tetra species that hardly grow up to 2 inches.
Whether you like big or small fishes, be prepared to change to a bigger tank when the fishes you first keep have grown in size. Generally, an average of 15 square units of surface area may accommodate one unit of body length of aquarium fish (tail excluded). You can calculate the sizes of the tank required for the maximum quantity of fish you decided to keep. For example, if you start with 5 goldfish 1” in size in a 10-gallon tank, be prepared to buy a 40-gallon tank when they grow to 3” in size and so on. It is very cruel to keep the grown-up fish in the same 10-gallon tank although they may survive with proper aeration and filtering system. They must have enough space to swim freely to be healthy and free from diseases.
Keep a good aquarium book as a reference on the types of fish suitable for a community tank.