How many fish can I put in my tank ?

Golden perch header white text web web

How many fish for farm dams.

How many fish to stock in a commercial pond.

Aquaponics

Determining Fish Stocking Density in Tank-Based Aquaculture Systems

There is no simple answer to this question.

Introduction

Tank-Based Aquaculture Systems and RAS

Usually, this topic relates to tank-based aquaculture systems, such as Recirculating Aquaculture Systems (RAS).

Factors Influencing Stocking Density

How many fish a tank can hold depends on the system itself and the skill of the operator, not the amount of water.

The main issue is how the filtration system manages the waste produced by the fish and whether the system can sustain appropriate oxygen levels. AND the operator's ability to manage the system is also crucial.

Practical Approach

The best way to determine the ideal stocking density is to add the number of fish you want and observe how you manage the system and how it copes with the biological load.

System Management

If the system fails to maintain high enough oxygen or safe ammonia levels, reduce the bioload by removing some fish or increase the biofilter's capacity to provide more space for beneficial bacteria.

It's the bacteria that do all the work in the system.

Requirements for Bacteria -

For bacteria to thrive and perform effectively, they need three things:

1 A place to live – sufficient surface area

2 Food – waste produced by the fish

3 Oxygen

(See the information about pH and ammonia)

Species-Specific Considerations

The number of fish that can be stocked varies depending on the species.

What you are actually doing now is your own research, specifically tailored to your particular system.

See this video showing jade perch stocked at extreme densities 42 seconds

Water Chemistry Tips

Here's a tip.

While you monitor the system, keep the pH below 7.0.

Best between 6.80 and 6.90.

Find out why here.

Behavioural Observations

Also, if the stocking density is too low, perch can start to pick on each other.

If you see red marks on your fish that are about the size of the fish's mouth, they are bickering (fighting) between themselves.

They are signalling to other fish to move on, go away, or find another place to live.

Adding a few more fish can stop this behaviour.

Likewise, removing a few fish can also resolve it.

For example, jade perch are much better stocked at high densities.

They will continue to grow (unlike silver perch) when kept at high densities.

Understanding Biomass

Let’s talk about biomass.

Just a simple explanation and how it relates to the question: biomass refers to the total weight of fish in the system, not the plants.

When you stock fingerlings, your system will handle the load well because they are small and their biomass is low.

As the fish grow, their biomass increases, putting more pressure on the filter bacteria to cope with their waste.

The amount of waste produced is directly related to the amount of food you feed each time.

Some commercial systems measure their capacity by the amount of food that can be added.

If the filter cannot keep up, ammonia levels will rise.

At the same time, growing fish require more oxygen, so the system must supply enough oxygen for both the fish and the bacteria as their demand increases.

Optimising System Capacity

The capacity of the filter can be increased by adding more area of bio-medium, allowing more bacteria to grow and handle the increased waste – this is ideal.

For backyard home systems, add another tank if you are using IBCcontainers.

For commercial facilities, increase the available surface area for more bacteria to grow.

If oxygen levels are too low, increase aeration in the system, (Or if you are using pure oxygen increase the flow rate.) especially after the filter and just before the water enters the fish holding tank.

Remember, the bacteria in the filter consume a lot of oxygen.