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  • Writer's pictureStewart Kessel

How Many Fish Can I Keep in My Aquarium

Updated: Jan 8

Importance of the Aquarium Fish Stock Level

The question 'How many fish can I keep in my fish tank?' is such a common question for both beginner and experienced fishkeepers alike, and it is such a vital one. Put simply - it can be the difference between a successful aquarium and a fishy grave to all your beloved inhabitants.

Overstocking and mixing the wrong fish (more on this later) in your aquarium will adversely affect the wellbeing of your fish and significantly increase the likelihood of disease. Given this danger, the best answer is always to be conservative with the number of fish in your tank.

"Remember, it is your moral and legal duty as a fishkeeper to look after the wellbeing of your aquarium inhabitants".
Shoal of Cardinal Tetra - Aquarium Stock Level

How to Calculate the Optimum Aquarium Fish Stock Level

We all want to keep as many fish as possible, but what is the rule to follow to have the safest number possible in your aquarium, whatever its size? Unfortunately, it is not that easy. While there are several guides, no one rule allows you precisely to calculate the ideal number of inhabitants for your aquarium.

Fisheries will often look to the kg weight of fish per volume of water, but this is somewhat impracticable in an aquarium. Who knows how much, for example, their neon tetra or angelfish weigh? A more commonly accepted approach calculates the maximum possible combined total length of your fish against the volume of water in your aquarium – a guide of 1 inch per gallon of water is typically applied.

1 inch per Gallon of Water Guide

While this is the most commonly used approach, it has also been oversimplified and has several flaws that you must consider alongside the calculation. Let us take you through the process and point out the potential pitfalls.

Firstly, you need to calculate the volume of water in your aquarium. The full capacity (gross water-volume) of your fish tank was probably quoted either in gallons or litres when you initially purchased your aquarium. Don't worry if it was not or you have forgotten as you can easily calculate if you measure the length, height and width of the tank – shape is important too. Use our online fish tank volume calculator to help.

Example: Typical rectangular fish tank.

length 60 cm x width 30 cm x height 40 cm = 72,000 cm3 (cubic centimetres)

72,000cm3/1000 = 72 litres (there are 10000 cubic centimetres per litre)

72/4.546 = 15.84 UK gallons (there are 4.546 litres in a UK gallon)

Gross water-volume = 15.84 UK gallons

However, this full capacity will be inaccurate for your furbished aquarium as it does not take into account the displaced volume due to substrate, décor – rocks, wood etc. Although this will depend on your specific set-up, typically, the furbished aquarium's net water-volume will be approximately 85-95% of your fish tank's gross water-volume (full theoretical capacity).

Based on being conservative with stock levels, it is best to use 85% in most cases. Although bear in mind that if your aquarium has large amounts of, for example, rock hardscape, then the figure could be considerably lower.

Example: Continuing with our previous example.

Net water-volume = 0.85 x 15.84 UK gallons (gross water-volume) = 13.46 UK gallons

Therefore, we now have the first component of the calculation, the tank's net water-volume, and now can calculate the theoretical total length of fish suitable for your aquarium. The guide is 1 inch of fish length to 1 UK gallon of water. So, the value in UK gallons is the total length in inches of fish that can be kept in that aquarium.

Example: Continuing with our previous example.

Guide to the total combined length of fish = 13.46 inches (1 inch for every UK gallon)

So, is it that simple? Sadly not, as mentioned in the introduction, several flaws and caveats must be considered when using this approach.

Length of Fish

For the length of fish, the most common mistake is not considering the adult size of the fish – it is not the size that you purchased the fish but their adult size that matters.

The myth that a fish adapts to their aquarium size is simply not correct. The truth is fish placed in an aquarium of inadequate size suffer ill health that can stunt their growth and reduce their life expectancy.

Another consideration is not all fish have the same body shape, and some are fuller-bodied than others. Therefore, in reality, the fish length should not be the only consideration.

Therefore, you must research and total-up the combined total adult size of all the fish on your livestock list and use your judgment to factor-in their relative body mass. If your chosen fish are large-bodied or, for example, are tall in comparison to their length, then you should downwardly estimate your total stock level.

Fish Species

You also need to consider the specific habitat needs and the temperament of the fish on your list. It is no good meeting the stocking guidelines if you house fish that require a different environment or are territorial and may bully or even eat the other inhabitants!

The key is thorough research to ensure that all the chosen livestock have the same basic requirements – water chemistry, temperature, water flow, and also that they are suitable to be kept together.

Fish also frequent different levels within the water column. For example, many catfish are bottom dwellers, whilst most tetra species swim in the upper water levels. Therefore, it is wise also to consider this when planning your stock.


The stock level assumes adequate filtration to meet the bio-load for the aquarium size. I would recommend a filter with a flow rate that turns over the water volume at least four times an hour and has sufficient filter media capacity for the aquariums water volume. A reduction in the stock level will be needed if using a smaller filter (not recommended).

Surface area

Both these fish tanks have a theoretical gross water volume of 72 litres, but there is a vital difference.

Fish Tanks of same volume but different surface area

The second taller aquarium has a significantly smaller surface area. This difference is crucial as it will dramatically reduce gas (oxygen and carbon dioxide) exchange at the water surface, vital for the wellbeing of your fish. This issue can be partially alleviated through methods to improve flow at the water surface. However, you also need to consider that the limited swimming space due to the dimensions is unlikely to be optimum for fast swimming fish. Again, you should factor this into your stock decisions.


We should not underestimate the importance of not overstocking. The guide of 1-inch fish length per Gallon of water helps lead us to a practical stock level, but only if we consider other stock considerations such as habitat requirements and compatibility of the inhabitants. Remember, it is also advisable to introduce and build up your fish stock over time slowly.

Author: Stewart Kessel CChem, MRSC

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