A large aquarium can be home to quite a number of breeds. But while you’re shopping at a pet store, you need to consider whether a fish breed is compatible with the ones you already own, or even the size of your aquarium.
Some fish thrive in water with currents, others will be stressed out by it. You also need to consider water temperature, and whether the water should be alkaline or acidic. Pick breeds that thrive in similar water conditions.
Some fish breeds are very aggressive and will “hog” all the food, thus depriving the more timid breeds of their share of the fish feed. You may try to add more food but that you run the risk of overfeeding the aggressive fish or contaminating the aquarium with uneaten, rotting food.
Aggressive fish may also have territorial disputes, also depriving timid fish of hiding places. The stress will make them sicklier.
Some fish are “loners” that will be stressed out in a full aquarium. Others are used to being part of a school of fish, and feel safer in numbers. Considering this aspect of the breed will give you an idea of how many fish you need to buy per breed, or the number of fish you can fit in an aquarium—obviously a a “loner” will need more space and a quiet hiding place.