At Aquacadabra, the question our experts get asked more than any other is ‘how many fish can I have in my tank?’. Unfortunately, while we would love to give a super simple answer to all fishkeepers, it’s not quite as easy as that. Finding the right balance to help your aquarium ecosystem thrive takes care, time and attention, and while we can’t promise a number of fish can be safely kept in a tank of a specific size, we can certainly help you find your own balance.
In this easy-to-follow guide, we break down the main rule of thumb that many fishkeepers use when initially finding their aquarium balance, as well as explain how to make this more accurate to your tank by considering extra factors like fish species, water quality and more. We’ve also included some top tips on how to tell if your tank is overcrowded, what dangers this could pose to your fish, and steps you can take to improve this. So, whether you’re stocking your first tank, or considering adding more fish to an existing aquarium, make sure you do it safely by following our advice.
The ‘one inch per gallon’ rule
Loved and hated at the same time, the most commonly-used rule for fish tank stocking is the ‘one inch per gallon’ rule. This somewhat crude measuring system advises that for every one gallon of water in your tank, you can have one inch length of fish. By this rule, in a five gallon tank (around 20 litres) you can have a group of fish whose combined body length equates to five inches. Other common sizes and their corresponding fish numbers include:
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How many fish in a 10 litre tank? - 2.5 inches of fish
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How many fish in a 15 litre tank? - 4 inches of fish
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How many fish in a 20 litre tank? - 5 inches of fish
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How many fish in a 25 litre tank? - 6.5 inches of fish
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How many fish in a 30 litre tank? - 9 inches of fish
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How many fish in a 40 litre tank? - 10.5 inches of fish
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How many fish in a 50 litre tank? - 13 inches of fish
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How many fish in a 60 litre tank? - 16 inches of fish
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How many fish in a 100 litre tank? - 26 inches of fish
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How many fish in a 120 litre tank? - 32 inches of fish
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How many fish in a 200 litre tank? - 53 inches of fish
Unfortunately, for something that is designed to help support aquatic life, the ‘one inch per gallon’ rule has been simplified to the point where it could pose potential problems for your tank. Not only does it only take into account two variables (tank and fish size), but these are inaccurate in themselves (as we’ll get into). It also fails to consider additional factors such as swimming space, territory demands, fish waste levels and oxygen levels, leaving too many things unaccounted for.
It’s because of these issues that our recommended method for figuring out how many fish you can have in a tank starts with the ‘one inch per gallon’ rule as a yardstick, before delving into each of these variables to adjust the number accordingly. To help you finish your calculations, we’ve broken down each variable, including how they impact the number of fish you can safely keep, below.
Adjustments to the ‘one inch per gallon’ rule
Every aquarium is different so, while all of these variables should be thought about before stocking your tank with fish, you may find that some of them don’t apply to you. To find out how many fish we would recommend you keep in your tank, simply identify the variables from our following list that are relevant to your individual fishkeeping needs.
Step 1 - Find the actual size of your fish tank
The first step in improving the accuracy of the ‘one inch per gallon’ rule is to accurately measure how much water your fish tank holds. This means not just taking the size of the tank displayed on the box or product listing, but actually working out the real volume of water.
You can do this by measuring the height, width and length of your tank and inputting them into our handy Aquarium Water Volume Calculator. This will then give you the maximum water capacity of your tank.
Top tip: We recommend measuring the height of your fish tank to around 2.5 centimetres (or 1 inch) shorter than the full height. This is because this is where your actual water line will sit, as you should never fill your tank up to the brim, instead leaving a small air gap between the surface of the water and the rim of the aquarium or cover.
From here, you should next calculate the net water capacity, which allows you to take into account lost water space that is taken up by things such as substrate, decorations, internal equipment, corals and plants. This will usually take up around 10% of the space inside your tank, so your net water capacity can be calculated from the following equation:
Maximum Water Capacity x 0.9 = Net Water Capacity
Top tip: When making use of this formula, you can change the 0.9 figure to suit your tank. So, if you have more decorations in your tank and only around 15% of the space is used, leaving 85% available for water, you can instead multiply your Maximum Water Capacity by 0.85 to get a more accurate Net Water Capacity.
Working example: Taking the Superfish Qubiq Pro 60 Aquarium as an example, which has the dimensions of 40cm width, 40cm length and 40cm height (37.5cm when accounting for the actual water line), and inputting it into our Aquarium Water Volume Calculator, we get a Maximum Water Capacity of 60L. Saving 10% of that for ornaments, gravel, fish and the built-in filter, we get the following completed equation:
Maximum Water Capacity x 0.9 = Net Water Capacity
60L x 0.9 = 54L
Superfish Qubiq Pro 60 LED Aquariums, £114.99
Step 2 - Find the actual size of your fish
The second step in improving the accuracy of the ‘one inch per gallon’ rule is to carefully estimate the maximum combined size of the fish you want to stock your tank with. Usually, this rule considers the full length of the fish, measuring from head to the start of the tail. This gives you a good starting point, but it doesn’t pay any attention to the height and width of the fish.
For slim-bodied shoaling fish such as neon tetras, you don’t need to worry much about overcrowding when following the ‘one inch per gallon’ rule, however, for full-bodied fish species like goldfish it pays to be aware. This isn’t due to the physical space they take up as much as it is the amount of waste they produce, as the more waste is created, the faster water quality will drop.
If you’re housing larger, heavy waste-producing fish in your tank, it would be better to take a more conservative approach to the rule. We recommend instead following a ‘one inch per 1.5 gallons’ if you’ve got some full-bodied fish, or ‘one inch per two gallons’ if all of your chosen species fit this profile.
Top tip: When calculating the length of your fish, it's also important to use the average size for fully-grown fish, not the juvenile length. This ensures that your tank will remain healthy and balanced even when all your small fries turn into adult fish.
Working example: Taking the Superfish Qubiq Pro 60 Aquarium again, which we know from the section above has a capacity of 54L, we can look at an example of how to calculate the actual size of your fish. Following the standard 1:1 ratio for inches to gallons, a 54L tank can support 16 inches of combined length. This would give you space for the following fish:
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x5 Cherry Barbs (Puntius titteya)
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x1 (male) Siamese Fighting Fish (Betta splendens)
The combination of a group of shoaling fish (Cherry Barbs) and a slightly larger feature fish (like Bettas) is perfect for medium-sized fish tanks like the Superfish Qubiq Pro 60. These species also pair well together as they are both freshwater species that prefer the same water conditions (a pH of around 7, with temperatures between 24 - 27°C) and that are not known for fighting.
It is, however, important to keep a close eye on your Cherry Barbs - they shouldn’t nip the fins of the betta as they’re one of the only barb species that doesn’t have this habit, but each fish will have their own quirks and behaviours so it’s always best to pay more attention to the community’s workings in the early stages.
In terms of length, these thin-bodied species fit perfectly into a 60L tank. The five Cherry Barbs (which have an adult max length of 2 inches) have a combined length of 10 inches, while the male Betta fish will grow to around 3 inches. This keeps you safely below the maximum 16 inch limit for this size of tank and, as neither species produces much waste, you won’t need to worry about the quality of the water too much either.
Step 3 - Double-check for extra risks
Last but not least, there are a few extra factors that you should be aware of that could impact how many fish you can safely keep in your aquarium. Fortunately, this is more of a safety check than anything that will change the number of fish you can keep, as there are other things you can do to reduce overcrowding risks for each factor. It’s mostly a case of the more you know, the safer your fish will be!
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Tank shape: Something which is often overlooked, the shape of your tank has a significant impact on the health of your fish, especially when it comes to finding a good livestock balance. This is because whether you have a short, rectangular tank or a tall spherical tank will impact the surface area of your water. Generally speaking, the bigger the surface area, the more oxygenated your water will be, meaning your fish will be healthier. If you do have a tank with a smaller surface area, make sure to invest in alternative methods of oxygenation, such as plants, gas lines or air pumps.
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Water filtration: Take into account your specific tank’s capabilities, such as its filtration system. Ideally, your system should be powerful enough to filter the total volume of water in your tank four times every hour in order to provide a safe environment for your tank’s inhabitants. You’ll also want to make sure that you’re not putting any unnecessary strain on your filter by keeping on top of a regular maintenance schedule that includes water changes.
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Incremental additions: How you introduce your fish to a tank is important, with making incremental additions being the safest method. Not only is this the best way of letting your fish adapt to their new surroundings whilst simultaneously creating a stronger environment to support fish life, but it also gives you a visual reference, allowing you to see how they swim, judge their stress levels and see if there is space for more.
Working example: Taking our Superfish Qubiq Pro 60 Aquarium for the last time, and working off the basis that we’ll be housing five Cherry Barbs and one male Siamese Fighting Fish in the 60 litre tank, we can run these last safety checks to make sure all the fish will strike a healthy balance.
Firstly, we know that the Superfish Qubiq Pro is cube shaped, measuring 40x40x40cm. This means that the surface area of the tank is big enough to give sufficient oxygen to the tank inhabitants we’ve named - so no issues here (though adding a wave maker or plants would still be a benefit!
Secondly, the Qubiq Pro comes with an integrated biological filter with a Crystal Clear cartridge. These are easy to replace and provide protection against excess algae growth while promoting clean, clear and healthy water - so again, no issues! All you need to do is make sure not to overfeed and maintain a good cleaning schedule and the filter will take care of the rest.
Lastly, for your incremental additions, the best approach is to introduce your Cherry Barbs first. As territorial shoaling fish, adding these all together to establish a community first will keep them happy and reduce any fighting over territory later on down the line. Your betta can follow next, ideally after your aquarium has had time to adjust to the chemical change caused by the barbs’ waste production.
How do I know if my fish tank is overcrowded?
With so many things to consider, there is no perfect science to creating a balanced aquarium, and things can go wrong from time to time. What’s important is familiarising yourself with the risks of an overcrowded fish tank, and learning to recognise the signs so that you can take action to improve the safety and health of the tank quickly.
The biggest risks for overcrowding, and how to spot and remedy them, are listed below:
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Poor water quality is the most common side effect of an overcrowded fish tank. Caused by too much fish waste, which drives ammonia spikes and toxic nitrate and nitrite levels, you need to perform regular water quality checks alongside your normal cleaning routine to make sure everything is well balanced. If there is a decline in water quality, using the corresponding aquarium additive or treatment will help to keep levels in safe boundaries while you address a more permanent solution, whether that be a stronger filter, better cleaning habits or removing some of your livestock.
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Limited swimming space will impact the happiness and physical health of your fish. Things like over-ornamentation, tanks with tall and thin designs and overcrowding can cause this to become a problem. We recommend paying close attention to the preferred swimming space of your inhabitants to make sure you get a mixture of top, middle and bottom swimmers so that you can maximise your space.
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Insufficient territory space can cause an increase in aggression, stress and fighting amongst your fish, which will negatively affect happiness and health. Territory battles can even result in injuries which, if infected, can lead to loss of your fish. Overcrowding exacerbates the problem as there are more fish fighting over space, but you can sometimes improve the situation by including more ornaments and caves. These act as territory markers, allowing fish to split up the space more easily, while caves give smaller fish a place to hide to improve their comfort levels.
Create a happy, balanced aquarium with Aquacadabra
When it comes to fishkeeping, hard and fast rules usually backfire so the best advice we can offer is to be cautious and consider as many elements as you can. Fishkeeping is a learning process, so starting small and working up within safe margins is an excellent way to ensure the happiness of your aquarium.
Find all the equipment you need to keep a successful fish tank, such as the Superfish Qubiq Pro 60 Aquarium we’ve used in our examples, right here at Aquacadabra.