How many fish in a tank?

Understandably one of the first things every fish keeper wants to know is, “How many fish should I keep in my tank? (or pond, or aquarium). 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…

how many fish in a tank
Tilapia grown at high stocking density.

Factors determining how many fish in a tank.

Acceptable fish stocking density will be determined by many factors not least of which will be affected by the fish keepers objectives. The following are, without question, some of the most important factors to consider

  • Water temperature – the lower the temperature the more fish can be kept due to a reduction in their metabolism. As a result of this the fish require far less oxygen. Significantly more oxygen can dissolve in colder water so there is a clear multiplying effect.
  • Water Quality – This is of utmost importance. More so than any other factor. Far more important than pond volume or surface area 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. Never try to fully stock a tank or aquarium from Day 1. It will always end in disaster. There are ways to help prepare the filter prior to stocking with fish by regular doses of inorganic ammonia. This technique of fishless cycling will allow the early growth of beneficial bacteria prior to the introduction of any fish. It does have limits however and is only a partial representation of natural fish waste. I would strongly recommend that initial fish stocking rates are kept really low , say less than 10% of design capacity, for the first few weeks even if these ‘fishes cycling’ techniques have been followed. Always, always be guided by frequent water tests over the first 14 days at least. Daily test would not be too frequent.
  • Feed Rate – the lower the feed rate the more fish can be kept. Fish dealers are well aware of this and give their fish a minimum ration. Hungry fish somehow always come across as healthy fish to the uninitiated. Its nice to see fish showing a definite interest in food but if the water starts to boil with fish clambering over each other they are definitely being underfed.
  • 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. Keep stocking densities much lower for fry and fingerlings.
  • 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 method to determine a safe stocking level. 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.

  1. 4 koi per thousand gallons
  2. 1 inch of fish per 10 gallons
  3. 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.

LOW10 – 20 (1m3 = 1,000 litres)
MEDIUM20 – 60
HIGH60 – 120

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./m3

Fish Stocking Rule of Thumb for Aquaponic Systems

Design Considerations

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;

  1. The size, or volume, of the tank or aquarium
  2. The mean weight of the fish
  3. 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.

1. Size of pond, tank or aquarium

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 – This is especially true of deeper glass aquariums. Not only is a great volume of required by the increase in water depth necessitates thicker glass to withstand the higher water pressures.
  • take up more space – Obvious really but something that is particularly relevant if the system needs to be inside a building.
  • require bigger pumps to provide the same turnover rates. (one hour turnover is a good recommendation for most ponds and tanks) The bigger the water volume the lower the exchange rate.
  • sometimes difficult to net fish. To drain a pond down and capture the fish can take all day. Compare this with a much smaller aquarium.
  • require greater quantities of medications when needed. These chemicals are often not cheap and many require repeat treatments.

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.

2. 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.

Key Points to consider

  • Water quality, as opposed to pond volume or water surface area, 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.

Recommendations for improving water quality.

  • 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 . A really simple and effective short term method of improving water quality. Longer term objectives would be to operate a more efficient water filtration system that allows optimum feeding rates at all times
    • Remove some of the fish. This is probably the best , but least popular, method of improving water quality. We naturally want to keep all of our fish. However, there is a limit to how many fish any system can hold. Go even slightly over this limit water quality will inevitably decline, the fish will become stressed, disease will take hold and fish may die.
    • Improve the efficiency of the filtration system. A very popular solution, much supported by the fish dealer would wants to sell you the latest ‘bit of kit’. Although this technique can be highly effective it does mean that the fish continue to grow and become even more dependent upon the new filtration system. When, not if, you have a mechanical breakdown you must have a suitable alarm system along with a simple and reliable automatic backup plan. Not a cheap option to install.
    • Carry out additional water changes. Provided there is a good source of high quality water available a water change nearly always is beneficial. Much better to have a continuous drip influx that can be fitted and forgotten about rather than large scale bulk water changes. Water quality will stabilise with such a system and it is easy to increase the flow when needed.

In conclusion

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.

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