The average 55-gallon salt water fish aquariums measure 47.5 inches long, 19 inches high and 12 inches wide inside, which yields a maximum capacity that is less than 47 gallons. When setting up a beginner salt water fish aquarium, it's important to remember that measurements are always taken inside of the tank, and inches translate to U.S. gallons. And then after the gravel and corals are added, there is more like 40 gallons remaining in the salt water aquarium.
Fish take up even more room. After looking at these figures, it might seem odd that manufacturers come up with such conflicting figures for their salt water fish aquariums. Beginner salt water fish tank owners should know that the 55-gallon measurement is referring to the imaginary capacity of the 48.25 x 12.75 x 21 salt water aquarium. Fish, water, corals, rocks and other additions are not figured into the measurement, which is actually just less than 56 gallons. Major aquarium manufacturers have apparently used these ostensible measurements for decades, for both fresh water and salt water fish aquariums.
Of course, it serves their purposes to sell a tank that reported to have a 55 gallon capacity, rather than saying it holds 47 gallons. Beginner salt water fish tank owners should not get discouraged, though; 55 gallons is plenty of room to start with. The only thing that aquarium owners must remember is that 40 gallons of water must be treated differently than 55 gallons of water when adding medications, so as not to injure the fish.
