Stocking your aquarium with colorful fish is exciting for beginner and experienced aquarium enthusiasts alike. It's also something that beginners do incorrectly. A lot of factors go into how many gallons of water you need per fish, but one generally accepted formula equates gallons of water to inches of fish.
How Many Gallons?
One general rule of thumb for stocking an aquarium is 1 gallon of water per every inch of mature fish. For example, if your newly purchased 4-inch goldfish has a mature size of 10 inches, not including the tail, she'll need a 10-gallon tank to herself. Always purchase a tank that will house your fish at their mature size; moving fish into a new tank, sometimes necessary, is stressful to fish. Saltwater species often need much more room than freshwater fish, sometimes at a ratio of 1 inch of fish per 2½ to 5 gallons of water. The measurement of the fish for the purposes of the fish-to-water ratio is the fish's length from mouth to the base of the tail; the tail itself is not included in the measurement.
More or Less
Some smaller, inactive fish may not require this much room, such as neon tetras. A small school of neon tetras can be kept in a tank where the fish-to-water ratio is two fish per 1 gallon. On the other hand, highly active fish who enjoy swimming all over the tank will need more room to be happy and healthy.
Adding to Your Tank
Adding decorations to your tank not only adds beauty, it's important for quality of life for many fish species. Some fish need hiding places such as dense plants, caves or other structures. Peaceful fish who are common prey animals in the wild, such as neon tetras, often seek refuge in plants and accessories. Territorial fish need caves and hiding places to call their own. While it's important to add these things, the number of fish you can keep in the aquarium changes with their addition, as the volume of water in the tank diminishes as you add fixtures. In some cases, though, you can add more fish when you decorate your tank: An open aquarium leads to fights for territorial fish, and decor gives security and boundary. Be careful of placement. If you have a mixed community tank with open-water swimmers and hiders, make sure enough free swimming space is available for your open-water fish.
Why It's Important
Stocking your tank with the correct number of fish is important to their health and quality of life. When a tank is overstocked, the amount of oxygen in the water becomes insufficient. More fish produce more waste, of course, which not only reduces the availability of oxygen but also can make the water toxic to the fish and prevent beneficial bacteria from doing their job. The notion that fish will "grow to the size of the tank" is simply not true. Many fish succumb to the hardships of an overstocked tank and will die off, one by one, until there is enough room for all. Some of the larger fish may become deformed, as their bone and muscle development are stunted by the lack of space.