There’s no golden rule for calculating how many angelfish you can put into your aquarium.
Instead, there are many variables that can make a difference beyond tank size, such as how often you change the water, the temperament of the fish, and the needs of other fish in the tank. However, tank size is a good place to start when deciding how many tropical angelfish you can keep in your tank.
There are three methods you can use to determine the fish capacity of your aquarium. Some methods will overstock your tank, and others may under stock the tank. Use these calculations as a guide in addition to your own intuition. The fish per gallon calculations are the easiest fish capacity calculations to use as there is much less math involved, however the floor and surface area capacity calculation is probably the best for the health of all fish.
For any of the fish capacity calculation methods, always calculate the adult size of the fish. The angelfish growth rate is fast and they can reach their adult size in a few months. To avoid having to buy and set up a new larger tank in only a few weeks, start your baby angelfish in their adult-sized tank.
A Good Method: The 1 Inch per Gallon Rule
This rule is a simple one to follow. For every gallon of water your tank holds, you can have a 1-inch fish using the length of the fish as a measurement. However, this does not take into account the height of fish or the space the décor and plants take up. Any gravel, aquarium decoration, and plants you add to the tank takes away from the number of gallons your aquarium actually holds.
Since angelfish are often taller than they are long, use the height of the adult angelfish, which can be up to 12 inches for scalare, the most commonly found angelfish in aquariums, and the less common altum. Then add about ten gallons of water to account for any gravel, décor, and plants in the tank.
For two adult angelfish, you need a tank at least 34 gallons in size using this calculation. (12 + 12 + 10 = 34). If you want a school of angelfish, you’ll need a much bigger tank. If you want a community tank with angelfish as the centerpiece, then you will definitely need a large tank.
At 1 inch of fish per gallon, the minimum size your tank should be:
2 angelfish: 34 gallons
4 angelfish: 58 gallons
6 angelfish: 82 gallons
8 angelfish: 106 gallons
10 angelfish: 130 gallons
A Better Method: The 1 Inch per 2 Gallons Rule
Some consider the 1 inch per gallon rule for stocking a fish tank to be overstocking a fish tank and asking for problems. To keep the water quality good without having to do water changes more frequently, you can use the 1 inch per 2 gallons rule. Since with this calculation you are calculating more space for your angels, you don’t necessarily need to add the 10 gallon buffer for gravel, décor, and plants.
With this rule the minimum gallons your aquarium would need to be:
2 angelfish: 48 gallons
4 angelfish: 96 gallons
6 angelfish: 144 gallons
8 angelfish: 192 gallons
10 angelfish: 240 gallons
Many experienced fish hobbyists consider the floor and surface area of the tank calculation to be the best for stocking a fish tank. The floor and surface area of a fish tank refers to how much floor space and water surface area there is. You only need to measure how long and wide the aquarium is, not the height. Longer and wider tanks can hold more fish, including angelfish, than short tall tanks.
The explanation: Oxygen exchanges at the surface of the aquarium, and bacteria processes fish waste at the bottom of the tank. The larger a tank’s top surface area, the more oxygen your tank will have cycling through the surface. With more floor space, the more fish waste your tank’s nitrogen cycle can process. Thus, according to Joseph Levine in The Complete Fishkeeper, “A long, low tank has a great carrying capacity than a shorter, taller one that holds the same amount of water.”
To make things more complicated, every fish species does not have the same oxygen requirements or produce the same amount of waste. The space needs of bigger and thicker fish (such as a goldfish) goes up exponentially. Smaller and thinner fish, such as neon tetras need less space. Angelfish are in the middle when it comes to oxygen needs and waste production.
How to Calculate How Many Angelfish Your Tank Can Hold Based on Floor and Surface Area:
To make things easy, the calculation given here is based on angelfish being large but thin fish and each angelfish needing 12 square inches of surface space.
Step 1: Calculate the surface area of your tank by measuring the length and width (but not the height), and multiplying the two.
For example, a 35-gallon tank may measure 24 inches long and 12 inches wide. The surface area would be 288 square inches. (24 x 12 = 288)
Step 2: Calculate how many inches of angelfish your tank can hold by dividing the surface area by the number 12. A midrange fish such as the angelfish needs 12 square inches of space per inch.
Surface area / 12 = total inches of angelfish your tank can hold
Step 3: Calculate how many angelfish your tank can hold by dividing the above answer by 12, the height of an adult angelfish.
Total inches of angelfish / 12 = number of angelfish your tank can hold
Using the example of the above 35-gallon tank:
An angelfish needs about 12 square inches of space per fish. Thus an aquarium with a surface area of 288 sq. inches could hold 24 inches worth of angelfish, or 2 angelfish. (288/12 = 24 inches, angelfish are 12 inches tall, so 24/12 = 2 fish)
Because you are using the height of the angelfish for this calculation, you are being conservative and will still have room for other fish to make a community tank.
Calculations done for you using the surface area calculation:
300 square inches: 2 angelfish
450 square inches: 3 angelfish
600 square inches: 4 angelfish
800 square inches: 5 to 6 angelfish
1000 square inches: 6 to 7 angelfish