Whether you use the fish per gallon or surface area calculations, none of the fish capacity calculations offer a perfect answer as to how many angelfish (or any other fish) you can stock your aquarium with. This is because there are many variables that affect the overall health of a fish tank, and the number of fish is just one component.
Angelfish are Tall
Fish tank stocking calculations typically use the length of a fish. However, angelfish are twice as tall as they are long and need extra height to swim and not damage their tails. When using any of the fish capacity calculations for angelfish, use their height to give them more space.
If you have just one bad tempered fish, whether an angelfish or other species, in the tank, then your tank is too small for that fish. It doesn’t matter if your calculations say you have under stocked your tank, an aggressive fish needs more space than the average fish. And angelfish fins and tails often become the first target of any bully fish. If there’s a bully in the tank, he needs to be moved to a larger tank or live a solitary life.
Fish capacity calculations are typically based on providing good fish tank care, and that includes regular water changes. If you do not change about 10% to 20% or your aquarium’s water at least twice a month, then you need to under stock your tank. If you want to over stock your tank, then you will need to increase those water changes to keep the water quality at healthy levels.
Aquarium décor, gravel, and even plants displace water. The more stuff you have in your tank, the less water it holds. So although you have a tank labeled as a 55-gallon fish tank, it does not really hold that much water once you’ve set up the tank. Take off 5 to 10 gallons when calculating how many angelfish your tank can hold to account for the décor and other items in the aquarium.