Ultimately, when stocking an aquarium, volume is just a small part of the equation. Of course, the dimensions of the tank will dictate which species are suitable, but how many you can keep will be determined by other factors. How cycled a tank is and how many water changes you want to do are often more important considerations. If you want a heavily stocked aquarium, you will have to have oversized filtration and cycle the tank properly in addition to doing lots of water changes. If you don’t want to do a lot of water changes, then understocking a tank (a few small fish in many gallons of water) may be the way to go. However, it’s not as fun to look at a tank with very few fish. Luckily, you can maximize the activity in your tank without adding to your list of chores by following  a few common sense guidelines. 

Biological Capacity

The biological capacity of an aquarium is how much waste or ammonia production the equipped biological filtration can handle. This is determined by testing for ammonia, nitrite, and nitrate. You can tell that a tank is over capacity when ammonia spikes, because this is the first form waste takes; when it spikes, the present ""good"" bacteria is not effectively able to convert it to less harmful nitrite and nitrate. The aquarium is essentially overloaded. This nitrifying bacteria will need more time to colonize and cycle the aquarium before you can safely add more fish. If the ammonia doesn’t zero out in the weeks after a spike, you will either need to increase biological filtration (more media or an added filter) or you may need to cut back on the number of fish. You may also need to do lots of water changes to keep ammonia diluted. If your tank is stable and ammonia and nitrite are zero and nitrates are low, this tells you that your tank can handle stocking a few more fish. This will be your best guide in building your aquarium. The ultimate number of fish you’ll keep in an aquarium is not as simple as...

“Inch per Gallon”

There has long been a myth amongst newcomers in the hobby that you can keep one inch of fish per gallon of water volume. This seems reasonable at first glance. The problem lies in the fact that this doesn’t account for the mass or girth of said fish. By following this logic, one might think that one 10-inch fish can live in a 10 gallon aquarium, which simply isn’t true. That fish would barely be able to turn around in a 10 gallon. So using the “inch per gallon” approach to stocking an aquarium is not a dependable way to go.

Schooling Fish vs. Solitary Fish

When planning how many of what fish to add to an aquarium, one should always take into account which species prefer to be in groups vs. living solitary.  If you plan on adding cory cats, for instance, you want to make sure to keep them in groups of at least six. Most tetras, danios, cory cats, barbs, rainbowfish, minnows, and some small loaches prefer to be in schools (6 or more). On the other hand, some fish prefer to be kept singly or one per tank (such as bettas, freshwater sharks, flying foxes, and some cichlids). There are also some fish that do best in species-only tanks (killifish, goodeids, piranha, other predatory fishes). You will be best served determining what schooling fish you would like to keep in your tank and adding these first. This will ensure they are comfortable and that your biological capacity isn’t overshot further down the road when you try to add 6 or more fish to a tank that may or may not be at its limit. If you decide to keep large predatory fish at some point, you may also want to cycle a tank with smaller prey fish before adding the main attraction. Predatory fish are often messy, ammonia-producing factories. 

What Part of the Water Column a Fish Occupies

Another consideration for stocking a tank is what part of the water column your fish will occupy. Aesthetically, the ideal setup will have fish occupying all parts (the top of the tank, the middle, and the bottom). There are different species of fish that prefer to occupy specific parts of the tank. Most catfish and loaches, for instance, lay low and occupy the bottom of the tank. Schooling fish such as tetras tend to hang out midwater. Then there are fish such as hatchetfish, killifish, guppies, splashing tetras, and pencilfish which tend to stay closer to the surface. If you add a small group (3-6 individuals, depending on size of tank) of surface-dwelling fish, followed by a small group of midwater fish, followed by a small group of bottom dwelling fish (each addition spaced out by a few weeks as your cycle and quarantine allows), then you’ll maximize your enjoyment of your tank without overstocking. But speaking of overstocking...

Overstocking on Purpose

In the case of particularly aggressive fish, such as African rift lake cichlids, you may want to purposefully overstock your tank. Be mindful in doing this and keep tabs on your water chemistry. Because a cichlid tank needs to be overstocked to avoid fish singling each other out, you’ll naturally run higher in the ammonia and nitrate department. This will also mean you’ll need to do more frequent and larger water changes. Typically a cichlid tank will be 55 gallons or bigger. You’ll want to start with six small fish to get the cycle going and avoid aggression. Then after it cycles, you’ll want to add no less than three fish at a time, as adding one or two to an established pecking order can result in the new fish being singled out, bullied, and sometimes even killed. A dozen fish or more in a large tank works out much better than a few fish in the case of rift lake cichlids. 

Conclusion

There’s infinite possible stocking options and combinations of fish in the home aquarium, which is a large part of what makes the fishkeeping hobby so fun. The sky’s the limit as to what you can do. However, there are still some pitfalls to watch out for (See Fish Compatibility blog). By paying attention to the temperaments and needs of the fish and by slowly stocking as things cycle (and as quarantine allows; see the Importance of Having a Quarantine Aquarium blog), you can have the aquarium of your dreams in a few months' time. It does take some patience, but it’s well worth the time and effort to have that beautiful aquarium to enrich your home.

Links of Interest

TFH magazine - https://www.tfhmagazine.com/articles/aquarium-basics/fish-selection-stocking-guide

Live Aquariua - https://www.liveaquaria.com/article/245/?aid=245