• First things first: Learn the nitrogen cycle.fishes image by Irena Hnidkova from Fotolia.com
  
  

Rule of Thumb

• A good guideline for determining the maximum number of fish to keep in any size tank is one inch of fish per one gallon of water.
  
  

Large Tanks are Different

• In order to create a healthy environment for your fish, add only 10 inches of fish (so roughly two or three) and wait a month. Doing this creates the nitrogen cycle, a process that establishes helpful bacteria in the water and removes ammonia, which, in abundance, will kill your fish. Test the levels of ammonia and nitrite daily; once they return to zero, which means they are balancing themselves out, you can add more fish.
  
  

Bottom Line

• The one-inch-per-gallon rule still applies, so be sure that you do not exceed 55 inches of fish in a 55-gallon tank. This roughly equates to four or five bottom-dwelling fish along with a couple algae eaters. For the happiest aquarium, buy community fish, those that coexist peacefully with other fish, such as rainbow fish, tetras or female bettas.