How many fish in an aquarium depends on the size and type of aquarium. A freshwater tank can support more fish than a saltwater tank of the same size and water capacity. It is important not to overcrowd your tank, since poor water
conditions and illness can result.
How many 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.
How many fish in a saltwater tank will always be a lower number than in a similar freshwater tank. 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.
These rules are only very basic guidelines, though. If you want a more precise way to calculate how many fish in an aquarium, you will need to know things such as the weight of your fish, the surface area of the water, or the capacity of your filter. Larger fish, of course, create more waste than smaller fish of the same length. Even if you have a 30 gallon aquarium, you have things like the filter, plants, decorations, and gravel taking up some of the space. If you know, 
or can reasonably calculate, the weight of your fish, stick to 1 gram of fish per 4 liters of water. Basing your decision of how many fish in an aquarium on surface area, do not exceed 1 inch of fish length per 12 square inches of surface area.
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. Regardless of how many fish in an aquarium, 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.
