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. Fortunately, this doesn't usually last long unless there is another conflict. After I acclimate and freshwater dip new fish, I place them in my collecting net described above and place them in the tank for up to a day prior to releasing them into their new environment. This appears to allow the residents to vent their aggression without harming the newcomers.