Frequently asked questions on stocking densities

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The number of fish you can safely keep in an aquarium is something that few experts can agree on. Matt Clarke explains why there's no easy answer to the question...

Why is it so important to think about stocking levels?

There is a limit to the amount of fish that can be housed in an aquarium. First, they need to have enough space to move about. It's no good trying to keep a fish that's 45cm/18" long in a tank that's only 30cm/12" wide. Second, the fish need enough space to coexist peacefully. Some fish are territorial, and when their territories overlap, fighting and deaths can occur. It's therefore vital that you give certain fish, such as cichlids, enough room to live alongside one another. However, the main reason is that they'll produce more pollution than the filter can handle.

What is a stocking density?

This is a guideline that lets you know the approximate number of fish you can keep in your aquarium (or pond). Although basic, such a calculation gives you a rough idea of whether you've got too many fish.

We recommend:

  • Tropicals: 1" per gal./ 2.5cm per 4.55 l.
  • Coldwater: 1" per gal./2.5cm per 4.55 l.
  • Marines (fish and inverts): 1" per 4 gal./2.5 cm per 18 l.
  • Marines (fish only): 1" per 2 gal./2.5cm per 9 l.
  • Ponds:10" per 100 gal./25 cm per 455 l.

How do I know how many fish to keep?

If you have a 91 x 30 x 38cm/36" x 12" x 15" tropical freshwater tank, calculate the volume by multiplying the length by the width by the depth (approximately 106 l./23 gal.) and then knock off about 10% to make up for the displacement caused by your decor. This gives about 96 l./21 gal.

There is a stocking density calculator on this PFK website.

The important part is that fish grow. Therefore, if you add the full stock in one go, by the time they've reached adulthood, you'll be hugely overstocked. Instead, use the adult size of the fish.

So using the above figures, I could add either 21 Neon tetras or one and a half Oscars, and still have room?

No. Stocking densities are very basic and they fall over when you start bringing other things into the equation - such as big fish, which place a greater load on the filtration system. Most of them only work when you stock small or medium-sized fishes. Space-wise, a 91cm/36" tank is too small for one adult Oscar.

Can stocking densities be exceeded?

Yes, but you are best trying to stock your tank so it doesn't go beyond the suggested stocking density, or you'll risk water quality problems. In aquaculture, stocking densities are tens of times higher than those we use in aquaria. However, you really need to know what you're doing if you want to exceed these recommended guidelines, so we wouldn't suggest exceeding them.

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

Stocking densities which use surface-area are based on the premise that the surface is important for gas exchange. These say that tanks with a smaller surface area can hold less fish than one with a bigger surface area. For example, a tank that's 30 x 30 x 60cm/12" x 12" x 24" tall only has a surface area of 0.09sq m/ 1.00sq ft. Whereas, a tank that is 60 x 30 x 30cm/24" x 12" x 12" has a surface area of 0.18sq m/2.00sq ft.

Although the two have the same volume, the tall one can only take half as many fish. I'd argue that if you have the same size filter on each tank, with the same amount of fish and the same feed rate, and you keep your water well-oxygenated, you'd be able to keep the same amount in each. I prefer the volumetric stocking guidelines we use in PFK.

So stocking densities are an oversimplification of something more complex?

Yes. Exactly. The actual amount of fish a tank can support is determined by its carrying capacity, but calculating this is complicated and beyond the average fishkeeper.

How does carrying capacity work?

Research by Hirayma used the surface area of the filter media, flow rate, number and size of filters, amount of oxygen consumed per minute, body mass of the fish, number of fish and the amount of food entering the system (and its protein content). Using a complex equation, you can find out the exact number of fishes the filter can support.

In a nutshell, a tank with a big filter with a high surface area and a hefty flow rate can break down more waste (and therefore support more fish) than one containing a small filter with a tiny surface area. If you add more fish, or feed them more, you'll place a greater load on your filter. Carrying capacity isn't a function of total mass, so a tank that can support 10 10g fishes cannot necessarily support a 100g fish.

There is a beta version of a carrying capacity calculator in the Fishkeeping Tools section of the PFK website.

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

Probably, but you'll also need to do a lot more maintenance to keep the nitrate level down. Nitrate accumulates quicker if you have more fish, feed them more, or use a higher-protein food. You can get an indication of the amount of nitrate that may accumulate using our Water Change Calculator.

What happens to the water if I get close to my carrying capacity, or exceed it?

If you add a few more fish, or give them extra food, or change to a high-protein diet, there will be a small blip in the ammonia and nitrite levels, and the nitrate level will start to increase. The raised levels will remain until the bacteria equilibrate with the extra pollution available. If you exceed the carrying capacity, the raised levels could remain indefinitely because the filter is not capable of removing the ammonia. A stocking density can give you a rough idea of how many fish you can keep.

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

Allow for growth, use a good filter with a high surface area, make regular water changes and test your water for signs of pollution.

Further reading

Hirayma, K. (1965) - Studies on water control by filtration through sand beds in marine aquariums with closed circulating systems: Oxygen consumption during filtration as an index in evaluating the degree of purification in breeding water. Bull. Jap. Soc. Sci. Fish. 31: 977-982.

Hirayma, K. (1966) - Rate of pollution of water by fish and the possible number and weight of fish kept in an aquarium. Bull. Jap. Soc. Sci. Fish. 32: 20-26.