How Many Fish Can I Have in My Aquarium?

How Many Fish Will My Aquarium Hold?

This is one of the most common questions in the aquarium hobby. Most often asked by new aquarists who can't help but want to enjoy every new fish they see. There are two factors at play when knowing how many fish your aquarium can hold. The first is nutrient load and the second is behavior. We'll discuss both below and how they effect your tanks' carrying capacity.

Carrying Capacity

Carrying capacity is an ecological term that describes the population or community limit an ecosystem can hold before food abundance, pollution, predation, disease, or other factors cause their numbers to stagnate or decrease.

Our aquariums are closed ecosystems and many of the same principles apply. While disease is certainly a factor in your aquarium, it does not apply to our definition of carrying capacity because the ecosystem is already small enough to more than easily pass disease if any is present. Two fish or two hundred, disease will spread relatively quickly in a closed ecosystem.

As for other factors like predation and food abundance, our closed ecosystems are also controlled, meaning we regulate which species we add and when we feed.

Nutrients

Therefore, in order to determine the number of fish we can add, we measure the two factors that most effect fish health which we can not prevent; nutrient levels and social behavior. By prevent I mean we can not prevent nutrients from being produced, but we can remove them after the fact. Same thing with behavior, you can't stop a territorial fish from being territorial, but you can curb their aggression by changing the habitat or removing the fish.

Bioload

Bioload refers to the amount of biological waste in your system. If you add more fish, your bioload increases.

Let's start with this formula in regard to nutrient levels and assume all fish behavior is ideal. If you perform one 25% water change once per week, or if you have a number of healthy, growing plants, and your nitrates are at 0 ppm right before your water change, or at any given time with plants, you are free and clear to add more fish in terms of your bioload. If your nitrates are 5-15 ppm with water change and or plants, you might consider a larger water change or increasing plant growth before you add more fish. If your nitrates are 15-20 ppm you are at carrying capacity in terms of maximum bioload and if your nitrates are above 20 ppm you already have too many fish.

Regardless of when you do your water change or how much you take out, your nitrates should be close to 5 to 15 ppm right before maintenance is done. If you have live plants and your nitrates are still above 20 ppm, try fertilizing micro-nutrients, adding CO2, or increasing the quality or quantity of light.

If you don't have live plants, try adding a biological filter media to your filter. Choose a porous media with a dense center that allows for aerobic and anaerobic bacteria growth. This article explains more about biological filtration.

Technically, if you wanted 100 fish in a 10 gallon, it would be possible by means of performing a 100% water change every few hours, but you would then be limited by your fishes' behavior, which leads us to the second factor.

Behavior

If your nutrient levels check out fine, but you are having aggression or nipping issues preventing you from adding more fish there are some things you can try to curb this behavior.

The first is to rearrange your tank. This breaks up territory and forces inhabitants to reestablish territory. This could be useful if you have added fish after older inhabitants had already established territory. Also, be sure to provide ideal habitats for each individual. If you have two fish that enjoy claiming caves for their own, make sure to build at least three caves for them to establish territory. Breaking up the tank and providing cover for fishes to hide when necessary can also be useful The best practice of course is to do your research to prevent adding problematic species.

Counterintuitively, if your nutrients check out, adding dither fish can curb aggression as well. Alternatively, you can remove the aggressor. However, this can lead to a new pecking order that does not necessarily solve your problem.

All this is to say, fish behavior is an important factor to consider when determining if you can add more fish. Their quality of life is determined by how realistic their environment is which includes nutrients, habitat, and tank mates.

You can read more about fish aggression here. 

Research

Make sure you have a solid understanding of your potential species selections might interact with one another before making a final selection. Have a contingency plan if things don't work out as expected.

Make sure your filtration is setup to accommodate the amount of fish you want. There is no reason you can't install an oversized filter, as long as you can control the flow. Design a setup that leaves you extra room to expand your chemical and biological filtration if necessary. Finally, make sure your aquarium size is appropriate for the fish you want.

And as always, if you have any questions about calculating how many fish you can add to your aquarium, don't hesitate to contact us.

Photos

• https://www.flickr.com/photos/gabrielsaldana/8946258983

• Boodleshire LLC 2022