Introducing new fish gradually and deliberately is important for a successful tank community. It is critically important, of course, to be sure your fish are compatible before introducing new tank mates. Keep in mind that fish, much like people, all have their own personalities and temperaments. Just because a particular species is supposed to get along with another doesn’t always mean they will. Introducing new fish carefully and in a planned way can help.
When establishing a new tank, start with two of the fish that are typically the most peaceful species on your list. Try to get the largest fish you can of this species. Next, introduce the next fish on your list, the second-to-most peaceful, in a pair. Try to find two fish that are slightly smaller than your first two, but still fairly large. Continue in this manner, with the fish size decreasing as the potential for aggression increases. This allows the least aggressive of the fish to establish their own territories in the tank, and also ensures that the more docile fish will be larger than the more aggressive fish.
Always make sure you have plenty of places for new fish to hide in your tank, so that they can become adjusted to their new environment slowly. You may want to rearrange the decorations in your tank prior to introducing new fish. Doing this can help distract the existing fish and force them to focus on something other than the new inhabitants. More important though, it changes all of the previous territorial boundaries in the tank. As the new fish become acclimated to their surrounds and try to establish their own territory, they will be on more even footing than if 
they are introduced into a previously stable environment.
When introducing new fish to an established community, it is always best to add more than one. Adding a single fish to an existing tank can cause territorial issues for the current inhabitants. While there may still be issues when two or more fish are added, the aggression aimed at the “new guys” will be dissipated among the newcomers.
Before introducing new fish in an established tank, always feed the current tank inhabitants. It is dangerous, not to mention unfair, to force new fish to compete with the established community over food.
Introducing new fish can be stressful to new and old tank-mates alike. Planning the introduction and proceeding in a deliberate way can help ease this transition.
