Community fish compatibility is important for a successful tank of community fish. While most fish classified as “community” will generally get along, there are definitely degrees of compatibility. And, as with any rule, there are always exceptions; some breeds of community fish will seem to get along well with certain species and not at all with others.
Community fish compatibility starts with finding out everything you can about potential fish for your tank. Although you are dealing with fish that are all classified as community fish, each species of fish has some level of aggression. Make a list of “candidates” for your tank, always trying to select fish that typically have similarly low levels of aggression. 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.
A general rule is that even fish with docile temperaments but who are very active can stress out other docile fish who are less active. Higher stress levels in the tank can lead to potential problems. Likewise, some community fish such as barbs and tetras respond better 
when added to a tank in a small school. Keeping the number of fish odd (that is, adding 3 rather than 2 or 4) seems to reduce aggressive behaviors.
Be aware that breeding can cause potential upset to your community fish compatibility. Adding mated pairs of Gouramis, Cichlids, or Angelfish to an existing tank can been very problematic.
When adding new fish to an established community, it is always best to add more than one. Even if you had no concerns about community fish compatibility before, 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.
