Arguably one of the most frequently asked questions by the new hobbyist is "How many fish will my aquarium hold?"

I love this question as there is no correct answer. Experienced aquarists will often answer, "you can just tell" - a great help to the novice! On the other hand, every time someone on a Newsgroup asks, they get this answer; "The rule of thumb is one inch of fish per gallon of water." Whilst this answer is not entirely incorrect, and may well be a useful guide, it is seriously flawed.

The first flaw is, how big is a gallon? Americans and Europeans measure this volume differently. Therefore the "rule" can immediately be broken. Secondly, the guide takes no account of the type of fish, the filtration systems employed, the frequency of water changes the amount of food entering the aquarium, the actual water volume and surface area of the aquarium and so on. These are all important factors in establishing how many fish will comfortably live in an aquarium.

The stocking level of an aquarium depends largely on the type of fish it will house. It is important that one researches the needs of any fish prior to purchase to avoid inappropriately housing a fish. Some fish need space for territories, grow particularly large or produce a lot of waste, in these cases fewer fish will be able to be housed in the tank. Other fish need room to shoal, whilst some spend much of their time resting so space must be apportioned accordingly. In any case, the aquarium should allow enough space for the fish to move freely and behave naturally.

Having said that, having a larger aquarium doesn't necessarily mean that it can hold more fish. Rather than water volume, surface area is the the dictating factor. Identical aquariums measuring 36 x 15 x 18 inches and 36 x 18 x 18 inches (length x height x width), even though the difference in volume is some 7.5 gallons, will hold the same number of fish as the surface area, 36 x 18, is the same for both tanks. This is why traditional goldfish bowls are frowned upon. The ratio of surface area to water volume is far too low for most fish.

The bioload i.e. the balance of waste control, should be another consideration. With a large number of fish comes a large amount of waste. Can your filters cope with the waste?

In the days before modern aquarium filters, aquarists managed waste and nitrate control with very regular (often daily) water changes. Now-a-days an aquarium should not normally require a water change or cleaning or more than once every week. The need to perform a water change relates primarily to how quickly nitrates in the water rise. If nitrates cannot be controlled with a weekly water change then something is a miss - probably too many fish. If your filters are becoming badly clogged with in two weeks, again excessive wastes are being produced. In short, the fewer fish you keep, the less maintenance your aquarium should need.

Feeding is another factor relating to stocking levels. The less food that enters the aquarium then the less waste is produced. At the same time however, all fish must be fed and if there is a lot of competition for food, smaller, slower or weaker fish may miss out. So once more, fewer fish helps towards a healthier tank.

Finally aesthetics. I feel that a large number of different types of fish looks messy and unorganized. You have your whole life to keep the species that you want so why keep them all at once? An aquarium looks at it's best if one can observe the fish behaving naturally without a mass of activity to distract the observer.

None of this has actually answered the original question, in essence every aquarium is unique and thus stocking levels are different every case. However, I hope I have made you think about the factors that limit the amount of fish an aquarium can hold and that you consider these next time you think there is room for just one more fish.