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There is an old rule of thumb that gives a very basic guideline that might help you until you gain more experience: "one inch of fish per gallon of water." In theory, using modern technology, it would be possible to exceed this guideline and add more fish. In reality, however, it is best to be conservative in this regard. Keep in mind that this "inch-per-gallon" guideline is for small, slim tropical fish larger, deep-bodied fish require more water per inch of length. Also, be aware that this guideline refers to the adult size of the fish, not the size of the fish at the time of purchase. Stores generally stock very young, small fish, which keeps prices low.

The most widely known rule for stocking a tank is one inch of fish per gallon of water. While this type of calculation works as a rough estimate, it leaves plenty of room for error. Like people, fish are not all the same size and shape. Stocking a ten-gallon tank with ten inches of slender shaped zebras is not the same as stocking it with ten inches of full-bodied goldfish. Larger bodied fish create far more waste, and therefore require more water volume.

A good rule of thumb is one inch of fish per gallon of water but there are exceptions to this rule. The guideline of an inch of fish per gallon of water is based on a standard rectangular aquarium of normal height. If you have an unusually-shaped fish tank, such as a hexagonal, or extra high aquarium, try keeping one inch of fish for every 12 square inches (length times width) of aquarium surface area. Remember to allow for growth.

The basic rule for keeping freshwater fish is one inch of fish per gallon to gallon and a half of water. No, this does not mean you can get yourself a ten gallon tank and put five 2 inch Oscars in it. This rule is based on the adult size of the fish you are keeping. It also changes for larger fish and depends on the aggression level and spawning tendencies of the fish.

A rough guide to the number of fish is to allow approximately 1 litre of water per centimetre of fish where there is good filtration. Allow 2 litres or more per cm for small to mid-sized goldfish and cichlids, or where filtration and/or surface area may be limiting. Allow more water (3-4 l per cm) as the fish grow larger, or where they are territorial.

The number of fish should be limited, as the new system cannot support full a organic load until the natural cycles have had a chance to establish. It is also important not to overcrowd the tank with fish. The basic rule is 1cm of fish per litre of water. Therefore, an aquarium of 100 litres provides enough space for around 20 fish sized 5cm. (Consider the expected final size of the fish.)