- The popular method to determine how many fish should be in an aquarium is an inch of fish to a gallon of water. However, saltwater has less dissolved oxygen than freshwater due to its density. An adequate amount of dissolved oxygen is required to keep the fish alive. The Aquarium Professional Group recommends keeping three inches of fish for every square foot of surface in a saltwater aquarium.
- A 55 gallon aquarium that is 48 inches long and 13 inches wide has approximately four square feet of surface area. Thus, 12 inches of fish can be kept in this environment.
- Tall and hexagon-shaped tanks of the same volume have less surface area than a basic rectangular tank. This causes these tanks to have less dissolved oxygen. Six inches of fish should be kept in a 55 gallon tank with an unusual shape.
- When setting up a new tank, the water must first go through a nitrogen cycle. This biological process stabilizes the system, making the water safer for saltwater fish. The majority of fish should not be added until after the tank is cycled.
- With thinner fish, such as eels, use a quarter of their body lengths as an inch measure. With thicker and taller fish, such as angelfish and puffers, estimate 1.4 inches of fish for every inch of body length.
- Young fish sometimes grow much larger than their original size. It is wise to stock a tank with young fish with only 75 percent of allowed inches to make room for growth.
