Misconception

• Many people believe that one inch of fish can fit in one gallon of water, so that 50 one-inch fish could fit in a 50-gallon aquarium. This calculation does not work for brackish water or salt water aquariums, or tall, narrow aquariums.
  
  

Size

• Many fish grow more than one inch across. Always use the size of the adult fish when making calculations about how many fish can fit in a particular tank. Some aggressive species, like the oscar or velvet cichlid, need 55 gallons for just one adult.
  
  

Considerations

• Gravel, plants and decorations take up space in an aquarium, leaving less room for the fish and less oxygen content in the water. Only add one or two fish at a time to a tank for a multi-species tank. If the fish start dying, stop adding fish.