Understandably one of the first things every fish keeper wants to know is, “How many fish should I keep in my tank? (or pond). This is another one of those ‘How long is a piece of string?” type of question, to which there is no simple and straightforward answer. It all depends…
Factors determining how many fish in a tank.
Acceptable fish stocking density will be determined by many factors. The following are, without question, some of the most important.
- Water temperature – the lower the temperature the more fish can be kept due to a reduction in their metabolism.
- Water Quality – This is of utmost importance. More so than any other factor. Far more important than pond volume for instance. The better the water quality the more fish can be kept. This is especially true of dissolved oxygen which is usually the most important water quality factor. Fish are far better off in a small, well oxygenated pond than a larger pond with indifferent water quality.
- Filter Maturity – Inextricably linked to water quality filters take time to mature. The bacteria need time to build up to cope with full load.
- Feed Rate – the lower the feed rate the more fish can be kept.
- Fish size – the smaller the size the great the number of fish that can be kept. However the total weight of fish that can be kept is lower. This is because smaller fish consume proportionately more food.
- Adult fish size – adequate space must be provided to allow all fish to reach a mature size. This will obviously vary from one species to another.
- Aesthetics – ornamental fish usually look better of they are not ‘bumping’ into each other. They are prized for their individual beauty and grace as they swim serenely around a pond. Having a lot of fish in a pond can just ‘look overcrowded’ even if the water quality is top notch and the fish are perfectly healthy.
To help illustrate some of the above, reputable fish dealers are perfectly able to keep large numbers of fish in a tank because they do not heat the water, keep feed rates to a minimum and do everything to maintain excellent water quality.
Rules of Thumb for ‘How many fish in a tank?’
Notwithstanding the above, there is still a desire for a straightforward answer to how many fish in a tank. This has inevitably lead to a number of simple ‘Rules of Thumb’, none of which provide a wholly satisfactory answer. The following have been used for many years by koi keepers.
- 4 koi per thousand gallons
- 1 inch of fish per 10 gallons
- 1 inch of fish per square foot of surface area
By comparison, aquaculturalists would find the above levels unacceptably low. Commercial fish farmers would consider the following stocking densities normal.
|LOW||10 – 20 kg.fish/m3 (1m3 = 1,000 litres)|
|MEDIUM||20 – 60 kg.fish/m3|
|HIGH||60 – 120 kg.fish/m3|
By way of an example, a koi keeper might calculate that a 1,000 gallon pond should house 100 inches of fish, for example, 5 fish each of 20 inches. If we assume a 20 inch fish weighs 2 kg. this would equate to a stocking density of around 2 kg./m3. A fish farmer, in comparison, would expect to stock 100 – 200 fish in the same tank!
Not surprisingly, aquaponic system designers tend to adopt stocking densities somewhere in the middle. Bernstein, for instance, recommends the following;
1 kg. of fish per 40 – 80 litres of water. This is equivalent to a stocking density of 12.5 – 25 kg./m3Fish Stocking Rule of Thumb for Aquaponic Systems
I think the above is a sensible starting point, all things considered.
Calculating how many fish should be stocked is an essential step when designing and running any kind of fish culture system. The three key aspects include;
- The size, or volume, of the tank
- The mean weight of the fish
- The stocking density of the fish – expressed as kg/m3
Knowing the above it is a straightforward calculation to determine how many fish there should be in tank. For example, if the tank is 2m3 (2,000 litres), the preferred stocking density is 20kg./m3 and the average fish weight is 0.2kg then the number of fish will be;
(2 x 20) /0.2 = 200 fish
This is all very well if you already have a good idea of the three parameters. Obviously, the size of the tank is usually easy to determine and doesn’t vary.
Generally speaking, large tanks are easier to maintain than smaller ones. Consequently it is usually better to err on the side of a bigger, rather than smaller, tank volume. However, large tanks can also suffer from problems of their own. For instance, large tanks are;
- more expensive
- take up more space
- require bigger pumps to provide the same turnover rates. (one hour turnover is a good recommendation for most ponds and tanks)
- sometimes difficult to net fish
- require greater quantities of medications when needed
Several smaller ponds allow better separation of fish size classes, each having their preferred food sizes and ration.
Generally speaking, a 1,000 litre tank is a good minimum size for an aquaponic system that is used to produce edible fish. Koi keepers often quote 1,000 gallons (4,500 litres) as a minimum size for adult fish.
Water depth may also play a part. I personally see little evidence to recommend depths over one metre. Almost invariably smaller fish thrive in shallower ponds.
Mean Fish Weight
Determining the average size of the fish is normally a straightforward process of netting, weighing and counting a representative sample. An accurate figure for how many fish are in a tank is vital and it is well worth physically counting them every so often. Recording any mortality is consequently also important. This information is indispensable if optimum feed rates are a key objective.
- Water quality, as opposed to pond volume, usually determines how many fish a tank can support.
- Dissolved oxygen is usually the most important parameter although ammonia and nitrite can also be significant.
- Overall water quality is determined by the amount of food given and the efficiency of the filtration system.
- Monitor water quality and fish behaviour regularly
- If waste begins to accumulate, or fish behaviour indicates a potential problem, consider implementing the following plan.
- Reduce the amount of food that is given
- Remove some of the fish
- Improve the efficiency of the filtration system
- Carry out additional water changes
One final point, building a larger pond is a much-loved recommendation by many fish dealers. After all isn’t that their business? However, it is rarely the best solution, especially in the short term. Improving filtration, and good stock control, will ensure that the carrying capacity of the pond is not exceeded. Happy fish, happy fish keepers.