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How many fish should you put in a fish tank?

A common question posed by fishkeeping beginners, hobbyists and fanatics alike, ‘how many fish can I have in my tank’ is a vital calculation that can mean the difference between a successful aquarium and a very short life for your fishy friends. Of course, we all want to keep as many fish as possible, but with overstocking presenting a major threat to the health and happiness of your fish, being conservative with the number of fish in your tank is often better.

In order to successfully keep a healthy and stable underwater ecosystem, you’ll need to follow a few rules, all of which we’ll be taking you through in this post so that you can work out the safest number of fish you can keep in your aquarium, whatever its size.

The one inch per gallon rule

Over the years, there have been many rules to help fishkeepers work out how many fish their aquarium can support, with the simplest being the classic ‘one inch of fish per gallon of water’ rule. While this familiar rule is widely known, it is a very crude method of measuring that has been heavily oversimplified for convenience.

One of the primary problems with this system is that it allows only for two variables, these being the length of the fish and the amount of water in the tank, when there are significantly more considerations to be made. If you’re seeking a hard and fast rule for how many fish you can keep, there isn’t one but, by using this method as a rough estimate, and by considering the other variables at play, you’ll be able to work out how many fish to keep in your tank.

For now, however, take the following numbers as your yardstick, then utilise the variables described in the next section to adjust as needed.

  • How many fish can you have in a 20 Litre tank? 5 inches
  • How many fish can you have in a 30 Litre tank? 7 inches
  • How many fish can you have in a 60 Litre tank? 15 inches
  • How many fish can you have in a 100 Litre tank? 25 inches

Main variables for calculating how many fish in a tank

Every aquarium is different so, while all of these variables should be thought about before stocking your tank with fish, you may find that many 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.

The size of your tank

While we know that one of the only variables the one inch per gallon rule considers is the volume of water a tank can hold, there is more to this calculation than simply reading the information that came with your aquarium. More often than not, the amount of water that your tank will hold is inaccurate, as it doesn’t take into account additional factors such as the space needed for gravel, decorations and plants. Fortunately, figuring out the net volume of your tank is a straightforward calculation.

Firstly, you’ll need to know the full capacity of your tank. Generally, this information will be available when you first buy the tank but the mathematical equation is incredibly straightforward, so don’t worry if you’ve forgotten. All you will need to figure this out is the height, width, length and shape of your tank which, with our handy fish tank volume calculator, will be able to tell you roughly how much water your tank can hold in litres or gallons.

With this figure, you can then work out your tank’s net volume. Generally, it's safest to assume that the net gallons/litres your tank can hold is around 85-90% of its full capacity. Mathematically, you should follow this equation:

Fish Tank Volume Calculator x 0.85 = Net Water Capacity


For example, if your tank measures 30x30x30cm, which makes your maximum water volume 27L, your net water capacity will be 22.95L.

The Size of Fish

The other variable the classic ‘one inch’ rule considers is the length of the fish but, unsurprisingly, this figure is also subject to inaccuracies; the most obvious being that fish are not all the same width and height. Goldfish for example, a particular favourite of fishkeepers, are a full-bodied fish species that will take up more space than other, thinner species. Furthermore, another commonly forgotten variable to consider is the adult size of the fish - especially if you plan to purchase juvenile fish - as one inch of length can quickly change.

When choosing which fish to stock your tank with, we would recommend you follow a more conservative estimate of one inch per two gallons for full-bodied fish and remember to take the estimated fully-grown length, to ensure a healthy aquarium balance that will last for years.

The Species of fish

While the size of your preferred fish is certainly important, a good fishkeeper should also consider other factors to ensure their aquarium does not feel overcrowded, such as the species of fish. Some species do not live happily together while others, such as schooling fish, will need to be kept in groups. If your fish are unhappy in their environment, their increased stress levels may risk lessening their lifespans - which is why we recommend fishkeepers take additional time to look into species compatibility, before purchasing any fish.

Your choices of species will also have an effect on how many fish you can safely keep in your aquarium, the number increasing when you find a good balance between top swimmers and bottom dwellers so they don’t get too cramped in their preferred area, for example. Similarly, you’ll want to consider whether your fish are particularly active as more swimming space will mean less fish can be kept.

Other Variables

Now that we’ve covered the main variables, there are some other things you should keep in mind while working out how many fish you can keep. These are unlikely to have as big an effect on your calculations as water volume and fish species, but the more you know, the safer your fish will be.

  • Surface Area: Something that has been excluded from the one inch per gallon rule is the effect the surface area of your aquarium has on the level of oxygen in the water. Generally speaking, the bigger the surface area, the more oxygen, which means you can keep more fish. The only time this is likely to affect your fish, however, is if you have a tall, thin tank with a very small surface area, in which case you could consider other routes to improving the oxygenation of your water in order to better protect your fish.
  • 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.
  • Incremental Additions: One of the safest ways to judge how many fish your tank can support is to use the inch to gallon ratio (with additional allowances made for the above variables) and add to your tank incrementally. 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, this will also give you a visual reference, allowing you to see how they swim, judge their stress levels and see if there is space for more.

Maximise the number of fish you can support at Aquacadabra

When it comes to fishkeeping, hard and fast rules usually backfire so the best advice available 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 a well-sized aquarium with a large surface area and a strong filtration system, online at Aquacadabra.