Choosing fish, one of your first decisions must be whether you will have a freshwater tank or a saltwater (also known as marine) tank. Saltwater aquarium fish are generally more beautiful and exotic-looking than freshwater aquarium fish. The trade-off, though, is that saltwater tanks require considerably more maintenance and knowledge than do freshwater tanks. Saltwater aquarium fish are also generally more expensive than freshwater fish, sometimes a lot more expensive. If you opt for a marine tank, it is a good idea to start with a few inexpensive fish until you have a tank well established.
Freshwater aquarium fish range from common species like goldfish to more rare varieties of tropical fish. With either saltwater or freshwater fish, not all species can co-exist well together, based on water temperature and other compatibility issues. There are hundreds of varieties of aquarium fish from which to choose; some popular categories of freshwater fish include catfish, cichlids, cyprinids, livebearers, and loaches. Popular marine fish include damsels, clownfish, and angelfish.
Before choosing fish, it is important to establish how many fish you should have. The number of fish you can safely keep depends on the size of your tank and whether you have fresh or salt water. 
In a freshwater tank, a good rule of thumb is to have at least one gallon or more of water for every inch of fish. If you have a 30 gallon tank, you should have no more than 30 “fish inches” of fish. In other words, you could have five or six fish that grow to five inches long or twelve to fifteen fish that grow to about two and a half inches long; either group adds up to less than 30 fish inches. {mosimage}Saltwater tanks require much more water space per fish, and the rule here is no more than two to four inches of fish per 10 gallons of water. Whether you choose a few good sized fish or several small ones is up to you, but using these rules will keep you from overcrowding your tank.
When choosing fish, be sure to check the compatibility of fish before putting them in an aquarium together. Aquarium fish are often classified as community, semi-aggressive, or aggressive fish. Community fish are often easiest to start with, since they generally get along with all other community fish. Some community fish, such as tetras and barbs, do best in schools. Keep three or more of these fish together, so that they will always have company.
Choosing fish is not an exact science, but there are important rules to abide by. Never put saltwater in fresh water, and vice versa. Choose fish that are compatible. Never overcrowd your tank. As long as you keep these critical rules in mind, choosing fish can be a fun adventure.
