The fish tank

Sep 3, 2020 · Aqu@teach

The basic components to consider are fish tanks, the sludge removal unit, the biofilter, the sump, plant beds, pumps, and piping. The function, required materials, and location of each of these, and their interaction with other components, all need to be considered. The interaction among the components, for example, will determine the number of pumps that will be required.

The fish tank will be the home of the fish for a relatively long period of time, so it should be chosen with care. The materials, design and size of the fish tank are all important, and should enable relatively easy observation and handling of fish, removal of solid particles, and good water circulation (simulation of natural water flow).

Volume

The volume of the fish tank depends on the following factors: (i) the number of fish it will have to house, (ii) the volume of the living space that each fish species requires, and (iii) the method of maintaining a stable water temperature. The design of aquaponic systems is based on the quantity of fish feed, which is related to fish density. The required volume of the fish tank is based on targeted fish density and biomass. For example, if the target density is 10 kg/m³, and it is planned to cultivate 30 kg of fish, a 3000 litre fish tank will be needed. One must also be aware that the fish will grow, and therefore the fish density and biomass will also increase during the production cycle. Generally, larger systems are more stable in terms of water temperature oscillations.

Figure 2: The importance of fish tank volume for water temperature oscillations: (left) small fish tanks exhibit faster water temperature changes; (right) in larger water volumes the temperature will be more stable

Shape

Fish tanks are usually either circular or rectangular. In addition, there are double-D or endless tanks that are a hybrid between circular tanks and long basins (Figure 3). Table 2 summarizes some general advantages and disadvantages of round, square and double-D tanks. In addition to these, other factors need to be considered, such as the type of fish species that one wants to rear. Bottom- dwelling fish such as burbot, turbot, sole or similar flatfish mostly stay on the bottom of the tank and may prefer a slow waterflow. Moreover, the bottom-dwelling fish may be stocked in such a way that the self-cleaning of the tank is actually achieved through fish movements and not the hydraulic pattern of the water column. Therefore, a square tank design may not be the worst solution for farming bottom-dwelling fish. Another aspect of tank design is the inclination of the tank bottom. While it has very little effect on the self-cleaning ability of the system, a higher inclination may help with draining the whole tank.

Figure 3. Different forms of fish tanks: (left) circular tank, (centre) rectangular tank (raceway or plug flow), and (right) double-D tank or D-ended raceway (hybrid of circular and raceway) (source: www.aqua-tech.eu, Bregnballe 2015 )

Table 3: Advantages and disadvantages of round, square and double-D fish tanks

Type of fish tank	Advantages	Disadvantages
Circular	Structural stability, no pressure points on corners Less material needed (cheap tank equipment cost) Conceptually simple Allow for homogeneous distribution of water and good water quality Flow conditions (centrifugal forces) wash the sediments towards the outflow in the centre of the basin centre towards the outflow (high self-cleaning effect) Low residence time of particles Oxygen control and regulation easy	Low area efficiency, low space utilization Hard to seal tank connectors (pipe through tank wall) Hard to segment Flow rates vary within the tank
Square	Efficient usage of area and space Easy to seal tank connectors Simple segmenting Easier to handle the fish	Low self-cleaning (possible dead zones, concentration gradients of dissolved oxygen and ammonia emerge) To prevent low self cleaning high flow rate needed High residence time of particles Medium oxygen control and regulation Pressure points in structure Feed waste is higher due to greater dispersion of the fish
Double-D	Efficient usage of area and space Water mixing partly possible Simple segmenting Medium self-cleaning Oxygen control and regulation easy The fish can swim in circles	Conceptually complex High amount of materials needed More expensive

Height and ratio

The fish tank should be at such a height that it allows staff to observe and work with the fish. If using deeper tanks, a window for observing the fish should be included and/or a stable walkway to access the tank. The height of the tank also determines the height of the water column and the rate of water flow to the next component of the aquaponic system (see Chapter 2).

Figure 4: Fish tanks positioned (left) above ground (photo: U.Strniša), and (right) at ground level (source: www.humblebynature.com/about-us/projects-at-humble- by-nature/aquaponics-solar-greenhouse)

If you are using a circular tank, you have to make sure that the water diameter/height follows a certain ratio. The maximum ratio should be 6:1. If the tanks are wider, then solids removal and even distribution of water from the inflow will be hindered. Reducing the ratio below 3:1 will create a vortex in the central drain, and oxygen will not be distributed evenly in the tank. Ratios below 3:1 should include a side drain (dual drain) to avoid the build-up of a vortex.

Materials

There are differences regarding investment costs, tank stability, and installation, but the most important thing is to make sure that the materials are safe for both the fish and the plants. This means that galvanized materials should be avoided, because of zinc toxicity. The wrong type of plastic can also be harmful to the fish. Thermally weldable plastics (so called thermoplasts such as PE, PP or PVC) are the best option, though they tend to be more expensive. The choice of plastic needs to take into account the following considerations:

UV resistance (black PE is UV resistant)
Porosity (PP is more porous than PE and therefore enables biofilms to grow)
Thermal stability (PVC becomes brittle below 0°C)

Because of its resistance to hard weather conditions, PE is the material to choose for long-lasting installations in greenhouses or outdoors.

Figure 5: Different fish tank materials: (top left) polyethylene (photo: U.Strniša), (top right) concrete (photo: U.Strniša), (bottom left) steel tanks covered with plastic liner (photo: ZHAW), and (bottom right) PVC tanks

Tank cover

Healthy fish are lively creatures and can jump out of the tank. All tanks should therefore be covered in order to prevent accidental losses and injury to the fish. Covers also prevent foreign objects from falling into the tank (Figure 6a). Tank covers reduce water losses due to evaporation and provide shading, which reduces overheating, prevents algae growth, and thus improves the wellbeing of the fish. In addition, most fish prefer to be in the shade rather than in direct sunlight (Figure 6b).

Figure 6: (left) A fish tank covered with netting to prevent accidental losses; (right) A tank liner and planted rafts prevent algae growth and provide shade (all photos: U.Strniša)

Water flow

Inflow and outflow

Ideally water should flow into the tank at an angle from above in order to enrich the water with oxygen and generate a circular flow in the tank (Figure 7a). If the water is oversaturated (oxygen saturation >100%, caused by the oxygenation units such as a low head oxygenator or oxygen cone), then the water should enter the fish tank below the surface through a perforated pipe (flute) which creates a circular water flow. The first perforation should lie just above the surface of the water and the total cross section of all perforations in the inflow pipe should be equal to the pipe’s cross section. The perforations also need to be smaller than the size of the fish that are kept in the system.

Figure 7: Examples of water inflow and outflow: (left) the water inflow is located above the tank at an angle; (right) the water outflow is in the centre of the bottom of the tank photos: U.Strniša)

The outflow of water from the tank should enable the removal of solid particles, while at the same time preventing the loss of fish; it is therefore usually placed in the centre of the bottom of the tank (Table 4). The correct dimensioning of the system and water flows prevents both clogging and overflowing. Each fish tank should be built as a separate hydraulic element, since hydraulic communication between fish tanks will end in total loss of all the fish if one pipe or one tank leaks. Therefore, every tank needs an option for overflow (Table 4). At ZHAW, we work with external standpipes or external overflows, so that structures within the fish tank do not interfere with fish handling procedures.

Table 4: Water outflow options (Source: Timmons & Ebeling 2007)

Type	(+) Advantages / (-) Disadvantages	Section
internal standpipe	(+) Water level control (+) No sediment deposition in pipeline (-) Disturbs netting of fish
External standpipe	(+) Water level control (+) Tank free of installations (-) Solids can settle in the pipe segment

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