How Many Fish Should I Have?

When purchasing new fish for an aquarium, almost every new aquarist hopes to achieve a beautiful underwater scene filled with tropical fish just like they see at libraries, museums, or hospitals.

Before deciding to purchase a tank and fill it to the “gills” with fish, it’s good to remember some important facts regarding the amount of living space they need.

1.  It is better to be conservative when stocking your aquarium. Fish get their oxygen from the water they live in. There is only so much oxygen dissolved in the water, and oxygen can only enter the aquarium at the water’s surface. If your fish have ample room, they will not only live longer and healthier lives, but you will save money in the long run.

2.  Fish that are overcrowded may never reach their total size capacity. Their growth rate slows down when the overcrowded population reaches the oxygen limits of the tank.

3.  The behavior patterns and aggressiveness of various types of fish are also a factor that can limit how many fish can be kept together. Allowing fish that are unevenly matched in aggressive tendencies to live together usually leads to a tank left with only one or two very happy fish.

4.  The size and species of your livestock is another factor to consider when stocking an aquarium. Larger species occupy more space, have larger appetites, and generate more waste than smaller species. Following is a good rule of thumb when considering different sizes of fish:

• Small to Medium Fish:  2 inches of fish per every 10 gallons
• Medium to Large Fish:  1 inch of fish for every 10 gallons 

This means that an 80-gallon tank can be stocked with 16 inches of small fish or 8 inches of large fish.

5.  Be sure to consider the effectiveness of your filtration system, the number of coral and invertebrates in your aquarium, and the actual volume of the tank once you have added rocks, sand, plants, and ornamentation.