As stated previously, FINS is an excellent reference. Once you understand the basic principles relating to keeping aquaria, such as the nitrogen cycle, the roles of bacteria, different types of filtration, physical parameters, and various chemical concerns, the next trick is learning who you can keep with who. These room-mates will literally kill each other. Obviously, if the opportunity presents itself for predation, the prey is lost. This is to be expected since your aquarium inhabitants spend most of their time in pursuit of a meal.
Aggression is another factor to consider.
It can be tough to guess how the different critters are going to get along. Some fish, such as the Beaugregory, will attack anyone and everyone. Some conflicts can also be a result of size differences. And at times it appears that some individuals just have a problem with other specific individuals. A common time for fish to display aggression is when new fish are introduced to the aquarium.