Correctly Sizing an Aquaponics System: Tank, Biofilter, Pump and Growing Area

Why correct sizing is crucial

An aquaponics system is a biological cycle. The fish population determines the amount of food. Solids and dissolved nutrients are created from the feed. Solids separation and biofilters have to process this load while the plants absorb some of the resulting nutrients.

The dimensioning should therefore start with the water volume, the planned fish population and the resulting daily feed quantity.

1. Determine the fish tank and stocking

For a small entry-level system, a fish tank with around 1,000 liters is a manageable starting point. Larger water volumes react more slowly to temperature changes, feeding errors and fluctuations in water quality than very small systems.

For beginners, a stock of around 10 to 20 kilograms of fish per 1,000 liters of water is a reasonable size. The upper limit should only be used when ventilation, solids separation, biofilters and monitoring are functioning reliably.

Pools under 500 liters should be filled with particular care. A maximum of around one kilogram of fish per 100 liters of water can be used as a conservative starting point.

A larger fish population does not automatically create a better system. First, it increases the oxygen requirement and the load on the filter and water cycle.

2. Calculate the daily feed quantity

The biofilter is not designed solely based on the pool volume. The most important thing is the daily feed quantity. The appropriate feed rate depends on the type of fish, water temperature, age of the animals and feed composition.

Daily feed amount = fish biomass × feed rate

With 15 kilograms of fish and an assumed feed rate of 1.5 percent, this results in:

15 kg × 0.015 = 0.225 kg or 225 g of feed per day

This value is used for planning. The actual feeding must be adapted to the fish species, temperature, growth and feeding behavior.

3. Remove solids in front of the biofilter

Fish droppings and leftover food should be removed from the water stream as early as possible. Too many solids in the biofilter or plant area can cause oxygen-poor deposits and affect water quality, bacteria and plant roots.

Depending on the design, settling tanks, radial flow filters, vortex filters or mechanical sieves are suitable. Finer organic substances can then be mineralized in a controlled manner.

4. Lay out the biofilter after loading

In the biofilter, nitrifying bacteria first convert ammonium or ammonia into nitrite and then into nitrate. To do this, they need a sufficiently large surface area, lots of oxygen and a steady flow of water.

The amount of filter medium depends on the daily amount of feed and its specific surface area. Manufacturer information on the permissible feed load should therefore be based on the planned maximum daily feeding.

A performance reserve is necessary because fish biomass and feed quantity increase during rearing. A new biofilter is also not immediately biologically efficient. Establishing a stable bacterial population usually takes several weeks.

5. Calculate the pump and water circuit

As a starting point, the fish tank water should be run through the treatment approximately once an hour. With a pool volume of 1,000 liters, this corresponds to around 1,000 liters of actual flow per hour.

The decisive factor is the flow at the real operating point. A pump with a nominal output of 2,000 liters per hour normally only achieves this value without significant head and pipe losses.

When selecting a pump, the following must be taken into account:

• the height difference to the highest outlet,
• Length and diameter of the pipes,
• elbows, valves, filters and branches,
• contamination during operation,
• a sufficient power reserve.

The pump characteristic curve must still deliver at least the required actual flow given the existing height difference.

6. Ensure ventilation independently

Fish, plant roots and filter bacteria consume oxygen. A separate air pump with air vents in the fish tank and, if necessary, in the biofilter is therefore an important safety component.

You should usually aim for at least 4 to 6 milligrams of dissolved oxygen per liter in the fish tank. Cold water species such as trout require higher values. Warm water can store less oxygen.

If the stocking is higher, a strategy for power failure or pump failure should also be in place.

7. Match the plant area to the amount of food

Too few plants can lead to rising nitrate levels. However, a very large plant area in relation to feeding can promote nutrient deficiencies.

Feed area rate = daily feed amount ÷ plant area

With 225 grams of feed and four square meters of plant area the result is:

225 g ÷ 4 m² = around 56 g of food per square meter and day

This planning value must be checked during operation based on the water values and plant growth. If nitrate rises sharply over the long term, more plants can be planted or the feed load can be reduced.

8. Consider sump and fluctuating water volume

With ebb and flow beds, the swamp must be able to absorb the displaced and returning water. It must not run dry when filling the beds and must not overflow when returning.

Additionally, evaporation, plant consumption and a safety reserve must be taken into account. The water level in the fish tank should remain as stable as possible.

Example of a small system

• Fish tank: about 1,000 liters
• Initial fish biomass: approximately 10 to 15 kilograms
• Upper fish biomass for beginners: a maximum of around 20 kilograms
• Example feeding: approximately 225 grams daily
• Actual water flow: approximately 1,000 liters per hour
• Plant area in the calculation example: four square meters
• separate ventilation
• Solids separation in front of the biofilter

These values are not rigid building regulations. Fish species, plant species, temperature, filter medium, system type and mode of operation may require other dimensions.

Control during ongoing operations

Even a well-planned system must be checked regularly. Particularly important are:

• temperature and pH value,
• dissolved oxygen,
• ammonium or ammonia,
• nitrite and nitrate,
• daily feed quantity,
• feeding behavior and health of the fish,
• flow and contamination of the filters,
• Root health and plant growth.

The calculation provides the starting point. Long-term stability is achieved through measurements, documentation and gradual adjustment of the system.