How aquarium water capacity relates to fish population

How Aquarium Water Capacities relates to Fish Population

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Longer is better than taller!

The first rule of thumb when buying a freshwater aquarium is to purchase the largest water capacity that you can afford. Tanks with large water capacities can house larger populations of tropical fish... and there much more forgiving than a small tank.

Wouldn't two aquariums with the same amount of water have the same fish population capacity?
The answer is no!

Let's look at two very popular aquariums for beginners. A 20 gallon long and a 20 gallon high. Both aquariums hold approximately 20 gallons of water... but one tank can hold more fish than the other.

The dimensions of a 20 gallon long is: 30"L x 12"W x 12"H
The dimensions of a 20 gallon high is: 24"L x 12"W x 16"H

To figure the surface area of any square or rectangular aquarium multiply Length x Width = Surface area.

A 20 gallon long has a surface area of 360 square inches.
A 20 gallon high has a surface area of 288 square inches.

What this means is that the 20 gallon long aquarium has an equal water capacity as the 20 gallon high... but the 20 long has a greater surface area, which relates directly to fish population capacities. Simply put... it can hold a few more tropical fish.

Here's why the 20 long can have a larger fish population...
The oxygen content of the water in a 20 long is much greater than that of a 20 tall because of surface area.
More oxygen means... more fish!

Oxygen and carbon dioxide exchange only occurs at the surface of the aquarium. Since a 20 long has a greater surface area... it has a greater exchange rate.

To introduce oxygen into the aquarium the surface of the water needs to be disturbed. Lakes and ponds are naturally oxygenated every time the wind blows. The wind kicks up waves disturbing the surface... which is oxygenating the water. In the aquarium we have to do this by mechanical means.

Mechanical oxygenation...
Mechanical oxygenation of the aquarium is very simple. Most aquariums will have a power head in each corner of the tank. If the water flow coming from the power head is directed towards the surface of the tank... it will disturb the surface water enough to create oxygen exchange. Most power heads also have an air tube that connects to them pumping room air into the tank. Very little of this room air is adsorbed into the water... but it does help create surface disturbance.

The mechanical filter attached to the back of the tank also provides surface disturbance as the filtered water is returned to the aquarium.

Now that we understand tank capacity and oxygenation... let's figure out how many fish we can keep in the aquarium.
As you can see we can keep more fish in the 20 gallon long, but how many fish can we keep in it?

The general rule of thumb for any freshwater tropical aquarium is 1" of body length per gallon of water. The back tail is always excluded from the measurement. This means we can keep a total of 36 inches of fish in the 20 gallon long. If the mature length of the fish you want to keep is 3", you can safely keep 12 of them in the tank.

This doesn't mean that you can keep 12 big fat 3" Goldfish in the aquarium... nor doe's it mean that you can keep two 18" Catfish in the tank. The biological filter could not handle the load created by the fish. In addition you couldn't provided enough oxygen for the fish to survive.