Owning a fish tank is very much like living in this big world. In some cases people do not fit in where they should and that creates a mess of problems most often. The same is true when you are the owner of fish. There are some fish that should not be put with others and as the owner of the tank you need to understand this concept when introducing fish to the tank.
There are two types of fish at this point, community and non community fish. The first type is fairly easy to understand. A community fish is one that can be introduced to the tank and will cause no harm to the surrounding fish. They are the type that are most often purchased and make for the best pets when you are looking to have a lot of fish in one tank.
Community fish are not hard to come by. Nearly all fish that are sold in the pet stores today are community based and can get along well with other fish of different species. While there is no law about it, pet store owners are asked to clearly label all fish in this case to allow the novice to know what is a community fish among their stock. If they are not marked then simply ask the clerk as they should know the answer.
Non community fish are those certain species that should not be put with other species. For one reason or another these fish just can not get along with any other fish then their own kind. This may be a territorial thing or it could be nothing more then the fish being a cannibal itself.
Many of the most well known fish in the world are non community fish. Gold fish and their many species are non community fish. People often times do not know that. There have been people who have spent a lot of money on a tank and some of the more beautiful fish and then introduced a gold fish. They wake up in the morning to find the other fish dead and the gold fish swimming happily around the empty tank.
The most extreme non community fish are those that cannot get along with their own kind. These fish are rare and many pet store owners will not carry them because storing a single fish in a tank is not cost effective. If you enter a pet store and see a single fish in a tank it is most likely a species that will kill other fish.
Know the difference between the two before you introduce a new fish to your tanks to avoid disaster.