Practical Fishkeeping Diploma - Week 5, Day 2

Here's the second of a five-part series of lessons to guide new aquarists through the basics of fishkeeping.

Week 5, Day 2: Aquarium size

The total mass of fish in a tank is called its stocking density. Optimal stocking densities for a tank rely on so many factors that accurate calculation is almost impossible. Among the influences on stocking density are: total water volume, fish mass, feeding amount and frequency, protein content of food, fish metabolic rate, aquarium surface area to volume ration, filter turnover, biological media volume, temperature, plant density, and supplementary filtration.

At purchase, few fish will have their full adult mass potential. For each doubling in length, mass increases many times over. A Goldfish of 2cm long may weigh around 0.2g, but 3g at 5cm. The larger fish, while only 3cm longer, is fifteen times the mass of the smaller fish. Increased fish mass will increase pollution.

When calculating how many fish a tank can hold, stocking density should be based around adult size of the fish being stocked.

Biological filter capacity is the principal limiting factor of stocking density. The more the tank’s ability to convert ammonia and nitrite, the higher the number of fish it can hold. In commercial systems, 30 to 40kg (and upwards) of fish per 1000l of water is commonplace. In aquaria, 2kg of fish per 100l may be at the filter’s limit.

A primitive way to determine stocking density is the cm (fish) per litre (water) formula. Based on the type of filtration used, and assuming that the tank will mainly house small fish with a low mass, the stocking rates can be calculated as follows:

Tanks with undergravel filters – 0.8cm fish per litre of water.

Tanks with internal canister filters – 1 to 1.2cm fish per litre of water.

Tanks with external canister filters – 1.5 to 1.8cm fish per litre of water.

Heavy filtration, high oxygenation, and frequent water changing combined may allow for an abnormally high stocking density

A ‘six-times’ rule is sometimes used to calculate minimum tank sizes for individual fish, determining that the length of the tank should be no less than six times the fish’s full adult length.

Territoriality should be factored in to tank size. Tanganyikan cichlids, for example, require a larger tank than peaceful fish of a similar size, to allow the formation of boundaries. Red tailed black sharks and Sucking loach are examples of fish that are peaceful while small, but become territorial as they age.

Some fish require a shoal, which means that their combined mass needs to be considered. Though they may be small, the weight of their numbers may require a larger tank.

Click here for next lesson: Substrate and decoration

How to gain your diploma: Once all the course modules and revision pages have all been posted online, we will open a link to a website that allows you to take your free online exam. If you pass the exam, you will digitally receive your very own Fishkeeping Diploma, to show that you have successfully completed the course, and which is yours to display on the wall near your aquarium, hang in your fish house — or keep somewhere safe where you can take it out and just look at it from time to time.

Note: The Fishkeeping Diploma is not a formal or accredited qualification and should not be confused with the type of diploma presented by colleges, universities and other educational establishments.

Further reading from our sponsors

Guide to regular fish care

Guide to fish nutrition

Routine aquarium care

Feeding your fish