Updated: Jun 6, 2022
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 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 plummet. If we do it right, 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.
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 building, 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.
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 X 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. If your nitrates are 5-10 ppm with water change and or plants, you might consider a larger water change or fertilizing your plants before you add more fish. If your nitrates are 10-20 ppm you are at carrying capacity 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 as low as possible right before maintenance is done. If you have live plants and your nitrates are still above 0 ppm, try fertilizing, adding CO2, or increasing the quality or quantity of your 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.
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.
You can read more about fish aggression here.
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