Whether you are a long time aquarist or a beginner, when it comes to adding new fish to an aquarium the main concern is compatibility. In a new tank with all new fish being added all at the same time, the occupants start off even. No "one" fish has become dominate yet. A few squabbles take place as territories are established, but once disputes are settled things quickly quite down. Every time you add a new occupant to an already established tank, the risk factor for the new addition to be harassed becomes high. You painstaking take steps to introduce the new fish into the community. Many times things go smoothly as expected, but when they don't, frustration can set in for both you and the new occupant.
When it comes to understanding fish and their behavior, there are no set rules as to how they are actually going to interact with other tank mates. Inevitably you will come across a fish that just "doesn't fit in", or goes against everything you have read about how it should act. All you've read says that it will or should get along with what you already have in your tank, but despite your research efforts to understand the nature of its compatibility, the fish has a bad attitude or its behavior completely goes against the grain. For example, how do you explain a 12 inch Peacock Grouper adopting a one inch Damselfish, treating it as its own offspring, while it then consumes everything else that moves in the tank that is small enough for it to eat? You can't! Only the Grouper and Damselfish know why, and they are not about to tell you their secret.
Here is another example; Yellow Tangs in nature are pretty...
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