Aquarium Stocking Calculator – How Many Fish Can You Keep?

Overstocking your fish tank can lead to serious problems such as aggressive behavior and poor water quality. 

How many fish in your aquarium can be calculated by using several tools, including an aquarium stocking calculator and aquarium bioload calculator. Basically, you need to know the correct fish-to-tank ratio for your tank. 

Worried that you can’t do the math? Don’t be! We’re here to help you work it out.

Keep reading to learn how many fish you can keep in your tank.

What To Consider When Stocking Aquariums With Fish?

Stocking Levels Aquarium And Fish Tanks

The traditional method for working out how many fish you can keep in your fish tank is “one inch of fish per one gallon of water.” However, that method is flawed, as it doesn’t take into account the requirements and behaviors of individual fish species. 

For example, some fish are highly territorial and won’t tolerate a crowded tank with limited space in which to establish a territory. Other fish need lots of open swimming space, while some produce a vast amount of waste that the filter system in an overcrowded tank can’t cope with.

The best way to work out how many fish you can keep in your tank is to use an aquarium stocking calculator. Simply insert the details about your fish tank shape and dimensions, together with information about the fish species you plan on keeping, and the calculator will tell you how many fish you can fit in your aquarium.

What Your Stocking Level Results Mean

Stocking calculators use the surface area of your aquarium to work out what the stock levels should be rather than using gallons. That works best because the surface area of the aquarium determines the amount of dissolved oxygen there will be in the water, and it’s that which determines how many fish you can keep.

For example, a tall, narrow fish tank can’t support as many fish as a regular rectangular tank, even though the water volume of both tanks is the same. That’s because, in the narrow tank, less surface area is in contact with the air.

Note that using air stones to agitate the water surface doesn’t mean you can keep more fish.

Research Fish Size

When you buy your fish, remember that most of the little creatures you buy in your local fish store are juveniles that may grow to double that size or more at maturity.

When calculating how many fish your tank will hold, always use the adult size of the fish for the calculation.

Tank Maintenance

The stocking calculator makes the assumption that the aquarium is correctly maintained. Regardless of the size of your fish tank, if you fail to maintain it correctly, you shouldn’t be keeping any fish at all!

So, be prepared to carry out partial water changes every week, vacuum the aquarium substrate to remove waste and uneaten food, and keep up to speed with filter media maintenance and replacement.

You must also install an adequate filtration system and monitor the water chemistry in the tank.

Exceptions To The Calculator Rule

Specific fish species can be exceptions to the calculator rule, depending on their individual requirements and preferences.

For example, species such as goldfish and Oscars produce a large amount of waste and are very oxygen-hungry, so they need more water volume than the calculation suggests. Aggressive species generally require more swimming space so that confrontations with their tank mates can be avoided. Most shoaling species need plenty of open water swimming space to thrive, and territorial fish also need extra room to adopt and defend a territory. 

Example Maximum Tropical Aquarium Stock Amounts

In the following table, you can see an illustration of maximum stocking levels for a single fish species. However, as mentioned above, you must research each species’ individual needs and temperament before stocking your tank.

Aquarium Stocking: Frequently Asked Questions

Here are the answers to some of the most frequently asked questions about aquarium stocking levels.

Q: Can stocking densities be exceeded?

Q: Why do some people use stocking densities based on surface-area?

A: The principle that ample surface area is essential for good gaseous exchange and, therefore, good oxygen saturation in the water is often used to calculate stocking densities.

Essentially, that calculation is based on the assumption that if you have a tank with a small surface area, you can keep less fish than you can in a tank with a greater surface area.

For example, a tall tank measuring 12” x 12” x 24” has a surface area of just 1.00sq ft. In comparison, a long tank that measures 24” x 12” x 12” has a surface area of 2.00sq ft.

So, although both tanks hold the same volume of water, the tall one can only accommodate half as many fish as the long aquarium. Personally, I would argue that provided you use an efficient filtration system with an appropriate GPH rate and the water is well-oxygenated, you could keep the same number of fish in each tank.

Q: How does carrying capacity work?

A: Carrying capacity determines the actual number of fish an aquarium can support. However, that’s a complicated calculation that most hobbyists can’t do.

In short, an aquarium with a big filter, a powerful flow rate, and a large surface area breaks down more waste than a tank that has a small filter and has a minimal surface area. Therefore, the former can support a greater number of fish. Each new fish you add requires more food, which, in turn, places more load on the filter.

Also, carrying capacity is not related to the total mass. That means a tank that can support ten 10g fish cannot accommodate one 100g fish.

Q: If I add an extra filter, can I keep more fish?

A: In theory, yes, if you add a filter to the tank or use a more powerful filter, you could add more fish to the setup. 

However, the more fish you have, the more food they need, so nitrates accumulate quicker. That means that you will need to perform more water changes to keep the water safe for your livestock.

Q: What if I get close to my carrying capacity or exceed it?

Q: What about the how many fish per gallon rule of thumb?

A: First of all, the “one inch of fish per gallon of water” rule only works for tropical fish species. Marine fish and coldwater fish need more space than that. 

Also, that rule of thumb calculation doesn’t take into account things like water displacement caused by substrate and decorations and space left at the top of the tank. The shape of the tank has a bearing on the calculation, as do the needs of the specific fish population that you want to keep.

However, for regular 25-gallon tropical freshwater tanks, the one-inch-per-gallon rule produces a similar result to the surface area calculation. Note that you must always take into consideration that the fish sold in fish stores are typically juveniles, and you must make the calculation based on the fishes’ adult size. 

Q: Why won’t more air stones increase the number of fish supported?

Q: Is there a more precise way to perform a fish stocking calculation?

A: In theory, the most precise calculation requires the gaseous exchange rate of the aquarium and the biomass of the livestock. To work out the biomass, you need to know the weight of the adult fish, and finding that data is challenging to say the least!

The best advice I can give you is to use the calculator above and, if in doubt, always understock your tank. 

Q: If stocking densities are flawed, how do I ensure I don’t harm my fish?

A: If you get your stocking densities wrong, ammonia levels in the water will increase, potentially poisoning your fish if the filtration system you’re running is unable to cope with the extra bioload. To get around that, you would need to carry out additional water changes every week, and filter maintenance tasks would have to be carried out more frequently. 

In Conclusion

I hope you found this guide to stocking levels in your aquarium helpful. If you did, please share.

The most accurate method of working out how many fish you can keep in your tank is to use an aquarium stocking calculator like the one we’ve included in this guide. Remember that you need to take into consideration the needs of the individual fish species, including territorial and schooling behaviors, the amount of waste produced, and aggression.

If you have any questions regarding stocking your aquarium, please ask them in the comments box below.