Is my tank overcrowded?

The question of how many fish can one put into their aquarium has crossed every fish keepers mind. But is there a way we can truely determine that? 

There are plenty of general guides of the amount of water volume per fish such as "10L per betta". Though this is an easy method to gauge the number of fish, this guide is typically to ensure easy maintenance by just water volume alone. There are plenty of factors that play into the decision of how many. 

To start, is the fish you intend to keep a schooling fish or solitary. This affects the amount solely due to the reason that solitary fish tend to be much more aggressive than the schooling type. These fish will fight and defend their territory, reducing the number of fish you can keep in the aquarium. Schooling fish on the other hand are more peaceful and work better in community settings. 

Next up, the size of the tank is another obvious limitation. We'd still like the fish to have space to swim. Keeping a large fish in a small aquarium would eventually lead to issues. Likewise, a large school of fish in a small aquarium can pose the same issues. 

Now, let's say we have the perfect sized tank and some community fish in mind. How many is too many? Some of us might like the look of a sparsely stocked tank whilst some prefer something a bit more filled. How can we ensure the health of the tank whilst achieving our goals? This is when an understanding of the water parameters and nitrogen cycle comes in. 

In order to maintain a healthy ecosystem in our tanks, we need the ammonia, nitrite and nitrate levels to be optimal. With our understanding of the nitrogen cycle, we can gauge whether there is too much fish in the tank by looking at the levels of the three compounds.

Levels of ammonia and nitrite should be 0 for a healthy aquarium, meaning there is sufficient colonies of beneficial bacteria to convert harmful waste products to harmless by-products. The addition of more fish should not cause spikes in ammonia or nitrite if the bacterial colonies are able to handle the additional bio-load. Hence, if the addition of more fish leads to an increase in ammonia and nitrites, it is more than likely that would be the maximum amount of fish you should have. 

The main way to work around this is to increase the colonies of bacteria able to do the conversions. This is the reason sumps are used in larger aquariums to increase the amount of filter media that can be held. The addition of sumps also increases the water volume, which in turn prevents any changes in the water parameter from being too drastic.