Keeping too many fish in too small a space can cause water quality issues (see – Keeping water, not fish!), and health problems for your fish.
Although some fish like to shoal, an overcrowded environment is unfair and unnatural for fish and may lead to territorial fighting and fin nipping, which in turn, can lead to disease outbreaks. On top of this, water quality will deteriorate causing more stress and health issues.
Needless to say, neither you nor your fish want to experience these stressful situations.
However, it isn’t fish quantity alone that affects the amount of fish you can safely keep. Another important factor in fish health is good oxygen levels. Aquariums that have a larger surface area have better gaseous exchange, therefore better oxygen levels in the water.
For example, a tall, slim 30 litre aquarium that has a small water surface will generally have poorer oxygen levels than an equivalent volume model with a wider surface opening.
The second example would be better able to support a more generous quantity of fish. Whatever the size, always ensure your aquarium has good circulation and consider adding an air pump to improve oxygenation.
It can be exciting when stocking an aquarium, and it’s easy to get carried away when choosing new fish.