Introduction
Aquaponics is an interesting concept, accessible to anyone. It is an eco-friendly, efficient, smart way of growing plants and fish, at the same time. This post, rather than focusing on the social aspects of aquaponics, will dive into a bit of biology.
What nutrients are given by fish to plants?
Ammonia is used here. It is in the feces and gills of the fish. It is also found in uneaten fish food and decomposed organic materials such as dead fish and leaves that have fallen in the fish tank. It is advised to add ammonia in the fish tank to kick-start the production of nutrients.
Ammonia exists in the water as unionized ammonia (NH3) and ionized ammonia (NH4+). Together, they are called the total ammonia nitrogen (TAN).
Ammonia starts the nitrogen cycle. This cycle is called nitrification. Ammonia is converted into nitrite (NO2) by the type of nitrifying bacteria called Nitrosomonas. Nitrite is then converted into nitrate (NO3) in the second nitrification step by another nitrifying bacteria called Nitrobacter. Nitrate is the form of nitrogen used by plants to grow and survive.
Above: Nitrosomonas
Ammonia is harmful to fish. Nitrites are also harmful to fish in large amounts. Nitrates are less harmful to fish, providing plants with excellent nutrients.
Above: The nitrification cycle
Biofilters are used in this process. Nitrosomonas and Nitrobacter are added to the filter. The filter has a large surface area where these bacteria grow. The physical shape can look like a board, gravel, or bio-balls. See the bio-balls below. They have lots of surface area so that bacteria can grow.
In a small aquaponics system, biofilters can be built in the tank. These are called internal filters, consisting of materials with a large surface area. Or they could be hang-on-the-back filers. These are placed on the back of the tank, partially inside and partially outside. Water is siphoned up to the filter, passes through the biofilter media, and then flows back into the tank.
In a large-scale aquaponics system, there can be separate chambers or tanks. Water from the fish tank is pumped into this chamber, passes through the biofilter media, and is then returned to the fish tank or directed to the plant-growing beds.
Obviously, nitrates are not the only nutrients plants need. Iron, potassium, molybdenum, and boron need to be added for the plants to grow. There are fertilizers specialized for aquaponics that include this. It should be kept in mind that aquaponics is the option that uses less fertilizer, than traditional farming uses. There are well-run aquaponics systems that do not use fertilizers at all.
pH in aquaponics?
An appropriate pH level keeps a healthy and functioning aquaponics system.
The pH scale, which stands for ‘power of hydrogen’, is a logarithmic measure used to determine the acidity or alkalinity of a solution, depending on the concentration of hydrogen ions (H+).
The level of acidity or basicity in a solution is determined by its hydrogen ion concentration. In acidic solutions, hydrogen ions outnumber hydroxide ions. Conversely, in basic solutions, hydroxide ions exceed hydrogen ions in concentration.
The Arrhenius theory, proposed in 1884, categorizes substances as acids or bases depending on their ion production in water. According to this theory, acids release a hydrogen ion (H+) in water, forming H3O+. The presence of a higher amount of H3O+ (or H+) ions signifies increased acidity of the solution. On the other hand, bases produce OH– ions, and a greater concentration of these ions indicates a more basic solution.
To quantify a solution’s acidity or basicity, the pH scale, ranging from 0 to 14, is used. This scale operates on the principle that a pH below 7 is acidic, 7 is neutral, and above 7 is basic. Lower pH values correspond to stronger acidity, while higher values indicate stronger basicity.
The pH value is calculated using the formula:
pH=− log[H+]
To measure a solution’s pH, several methods are available:
- pH paper, which changes color according to the pH and is compared against a color chart.
- pH testing kits, suitable for liquid solutions, also use a color chart for comparison.
- Digital pH meters provide the most precise pH readings automatically.
In aquaponics, as in hydroponics and traditional soil gardening, maintaining an appropriate pH level is crucial for the health of the entire ecosystem. This task is more complex in aquaponics due to the presence of three distinct organisms with varying pH preferences: plants, fish, and bacteria.
The pH of the soil is instrumental in determining nutrient availability and absorption by plants, thereby affecting plant growth. Optimal nutrient availability for most plants occurs in the pH range of 6.5 to 7.5, close to neutral. Extreme pH levels can adversely affect plant health, with very alkaline soils reducing the availability of micronutrients like zinc and copper, and very acidic soils diminishing the absorption of calcium, magnesium, and phosphorus.
Fish have their specific pH requirements depending on the species. Saltwater fish typically prefer an alkaline environment with pH values of 8.0 or higher, whereas freshwater fish do well in a slightly acidic range of 5.5 to 7.5. An ideal pH range for fish health is between 6.5 and 9.0. Deviations outside this range can stress the fish and increase mortality risks. Additionally, the water’s pH can influence the solubility and toxicity of chemicals and heavy metals, impacting fish health.
Bacteria, while generally adaptable, also have preferred pH levels for optimal growth. Most bacteria thrive best in a range of 6.5 to 7.0, although some can survive in highly acidic conditions as low as pH 1. However, these bacteria maintain an internal pH closer to neutral.
Given that plants often favor slightly acidic conditions, fish and bacteria lean towards a slightly alkaline environment, maintaining a pH range of 6.5 to 7.5 is generally optimal for aquaponic systems.
In an aquaponic system, the pH level is significantly influenced by the water’s hardness, which is crucial for its buffering capacity. This capacity is the water’s ability to sustain a consistent pH level, even when small amounts of acids or bases are introduced. In cases where non-distilled water is used, it’s likely to contain dissolved mineral salts that affect the water’s pH.
Water hardness is defined by the level of these dissolved minerals. The higher their concentration, the harder the water, and the lower the concentration, the softer the water.
Two primary forms of water hardness exist: carbonate hardness (KH) and general hardness (GH). Carbonate hardness is about the water’s ability to buffer or maintain alkalinity, while general hardness relates to the levels of calcium and magnesium ions.
In the context of an aquaponic system, carbonate hardness is more critical for establishing the system’s buffer capacity, essential for pH stability. This stability is crucial as sudden pH changes can be lethal to the fish and detrimental to the bacteria in the system. A low carbonate hardness could lead to a dangerously low pH, harming the bacteria.
For aquaponic systems, an ideal carbonate hardness level is above 4.0, ensuring adequate buffering for stable pH. This level can be assessed using a specific water test kit. If the KH level drops below 4.0 dKH, potassium bicarbonate can be added at a rate of 2 ½ teaspoons per 100 gallons of water to raise the dKH level, thus enhancing the system’s buffering capability.
Understanding the causes of pH fluctuations is crucial for maintaining a near-neutral pH in an aquaponic system.
Numerous external factors can either elevate or lower the system’s pH. A rise in pH often occurs due to an accumulation of carbonates, which can be attributed to the use of hard water. The type of grow media and construction materials of the tank or beds, such as limestone and concrete, can also contribute to an increased pH.
While a higher pH is typical in the early stages of the system, the development of a robust bacterial colony can gradually lower the pH. This reduction happens as the bacteria engage in nitrification, preventing carbonate buildup. However, excessively high pH levels can negatively impact plant growth and the development of fruits and flowers.
On the other hand, a decrease in pH is commonly linked to the nitrification process, where fish waste is transformed into plant nutrients, thereby increasing the water’s acidity. The construction materials of the aquaponic setup and the types of plants grown can also contribute to a reduction in pH. It’s important to adjust a low pH to safeguard the health of both the fish and the bacteria in the system.
In an aquaponic system, a buffer can maintain the water’s pH balance, but sometimes direct intervention is needed if the pH strays too far from the ideal range.
To address a low pH, increasing the water’s alkalinity is necessary. This can be achieved by mixing equal parts of calcium carbonate and potassium carbonate and then adding this blend to the water. Carbonates are the preferred choice due to their non-caustic nature and their ability to strengthen the carbonate buffer. Alternatively, simple hydroxides like sodium hydroxide can also be used to elevate the water’s pH.
If the pH is excessively high, it’s important to boost the water’s acidity to lower the pH. This can be done by introducing phosphoric acid, which is both safe and efficient. Besides adjusting the pH, phosphoric acid introduces phosphates, beneficial for plant nutrition. Another method to control high pH is using a reverse osmosis (RO) filter, which removes carbonates from the water, thereby purifying it and lowering the pH.
Additional Advice:
- Avoid using vinegar to reduce pH as it’s too weak, and steer clear of citric acid, which has antibacterial properties and can harm plants.
- Regularly monitor and fine-tune the pH level for the smooth operation of your aquaponic system.
- When making pH adjustments, do so gradually to prevent shock to the living components of the system.
In conclusion, effective pH management is essential for a thriving aquaponic system. All living entities in the system, including fish, plants, and bacteria, have their preferred pH ranges. However, maintaining a pH between 6.5 and 7.5 is generally suitable. Ensure the carbonate hardness (KH) is above 4.0, and add potassium bicarbonate if needed to maintain this balance.
Oxygen in Aquaponics?
Water in aquaponics systems contains nutrients and minerals, and dissolved oxygen that the plants, fish and bacteria require to carry out their functions. Oxygen exists in aquaponics systems in the form of Dissolved oxygen (DO). Fish need oxygen for respiration. Plants need it for respiration, health, the strength of their roots, and nutrient uptake. Bacteria also need it, for nitrification and respiration.
Different fish require different DO levles. Most grow and thrive within a DO range of 4-5 mg/liter. If the DO level drops below 4 mg/liter, fish may swim at the surface, breathe rapidly, and lose appetite. A 5 mg/liter would be required for warm water fish like bluegill, catfish, and bass. Coldwater fish like trout require about 6.5 mg/liter of DO to maintain good health and growth.
Fish such as tilapia can survive in lower DO, but their growth will be hampered.
High temperatures raise the metabolic rate of fish, making fish need more oxygen.
Plants also require oxygen. During respiration, plants release energy for root growth and nutrient uptake. They take in oxygen through stems, leaves, and roots. Plants usually require high DO levels, often exceeding 3mg/liter of dissolved oxygen.
What happens if the DO levels are too low?
- Root rots
- Increase of toxins, which leads to limited absorption of water and nutrients
- Loses a net amount of nutrients
- Calcium deficiency
When plants bloom flowers, they need more oxygen than their vegetative state. Temperature, nutrient uptakes, and the size of the plant’s roots are factors.
How can we know when plant roots do not receive enough oxygen?
- The plant gets droopy under warm conditions.
- Browning and dying roots
- Slimy roots
Bacteria need enough dissolved oxygen to do their task. Optimally, the levels of bacteria are 4-8 mg/liter. The nitrification process stops if the DO level goes under 2 mg/liter. It will still happen when the DO level is 3.0 mg/liter.
Other factors that can change the DO level include water temperature. High temperature holds less DO, and vice versa. Molecular activity is enhanced in warm water, pushing oxygen out of the spaces between molecules out of the water.
Low pH holds lower dissolved oxygen.
Pressure in the water increases DO levels.
Wind helps spread the oxygen within the water.
Algae consume oxygen, therefore decrease DO levels.
Uneaten fish food and other wastes decrease DO levels.
Small aquaponics rarely have problems related to dissolved oxygen.
However, controlling DO levels can be necessary in commercial aquaponics systems.
The first solution is using an air pump. Accompanied with air stones, it breaks down the oxygen into small bubbles.
The second solution is creating turbulence in the water.
Do not put too much fish, as they all consume oxygen, decreasing the DO levels.
Make sure the water is flowing. Cascading water leads to aeration. Water breaking into smaller droplets increases the surface area of water, leading to a frequent exchange of gases between the water and the surrounding air.
Ventilation
The definition of ventilation is opening the system to an outdoor space. This can be used to control indoor temperature, air motion, and humidity. Fresh air, wind, and sunshine can help, as they are often insufficient indoors. Alternatives also exist: fans for breezes; grow lights for sunshine; and watering systems for precipitation.
Humidity is controlled with effective ventilation. High humidity is caused in an enclosed enivironment, as evaporation occurs. Without control, plants pests and disease can be problems. Creatures such as fungi, bacteria, and certain insects enjoy humid conditions. Transpiration (the process in plants where water is absorbed by the roots and then evaporated from the plant’s leaves, stems, and flowers) is limited in high humidity. This is a problem since nutrient uptake and transport is limited, since nutrients are sent from the roots to the rest of the plant. It also damages the water balance. Transpitation consumes heat, cooling the plant. So when it is bothered, temperature is not properly regulated. The exchange of gases, namely letting carbon dioxide in and oxygen out, is also limited. This is because the stomata (tiny openings on the leaf surface) is not properly used when transpiration is limited.
Another reason why carbon dioxide is supplied through ventilation is the simple fact that new CO2 can be given from outdoors. Old air with an imbalance of CO2 and oxygen can suffocate plants.
An excessively high temperature is prevented through ventilation. This is helpful since grow lights often cause plants to heat up.
Little unexpected, stems and roots of plants are strengthened. Just like people’s muscles are strengthened through exercise, stems and roots are strengthened through wind.
So what can we do for ventilation?
One option is using an oscillating fan. It moves air around the room, blowing various directions like the wind. Oscillating fans are easy to set up. Depending on the size of the aquponics system, a few fans can be required.
Extractor fans can remove hot and humid air. Place them on the wall, and in a high position. It should be on the opposite side of vents.
Passive intake uses natural airflow and negative pressure to bring air into the room. It is leaving a path to let air in. Active intake pulls air into the aquaponics system using fans for high circulation levels.
Humidifiers and dehumidifiers are also a good way to help control the humidity quickly.
So what fans should we choose?
The CFM should be greater than the volume of the aquaponics system. CFM stands for cubic feet per minute. It shows how much air the fan can move in a minute.
Water Temperature
Water temperature should be preserved for the health of the fish, plants, and bacteria. It is also for the breakdown and uptake of nutrients and wastes.
These are some factors that affect water temperature.
- Capacity and consistency of source of heat
- Climate
- Air temperature
- Tank materials
- Exposed pipings and length of pipes
- Growing system
- Tank placements
- Insulation of the system
- Total water volume
These are some solutions for stable water temperature.
- In cold climates, insulate tanks, grow beds, pipes, and the greenhouse so that water heat can be preserved. Burying tanks in the ground is also a way for insulation.
- Use water or air heaters.
This is the optimum temperature range for aquaponics.
Fish:
- Tropical Fish: 71-89 °F (22-32°C)
- Cold-Water Fish: 50-64°F (10-18°C)
Plants:
- Most vegetables: 64-86°F (18-30°C)
- Some vegetables like lettuce and cucumber: 46-68°F (8-20°C)
- Other vegetables Like basil: 62-86°F (17-30°C)
- Leafy Greens: 78°F (26°C)
Bacteria:
- Grows in 62-93°F (17-34°C)
Heat stress of plants is a problem. The roots get shut and the plants go into survival mode. These are some symptoms.
- Wilting
- Low levels of DO
- Flowers drop, fruiting stops
- Roots get slimy
- Soft brown spots on fruits
- Roots turn black and die
Stable temperature keeps fish happy and healthy, makes them grow faster, lowers the chance of diseases, feed conversion becomes efficient.
Grow Lights
Lights help photosynthesis. Wrong lighting can stress the fish, limit plant growth, and kill bacteria.
In daytime, light helps plants produce energy through photosynthesis. At night, plants break this energy down through respiration for the growth and flowering process. 12-16 hours of light would work.
These are signs that plants get too much light.
- Leaves are limp or dried out with curled leaf edges.
- The plant looks weak.
- Leaves close to the light have dead brown spots.
These are signs that plant do not get enough light.
- Thin, uneven growth with tall spindly stems.
- The plant leans toward the light.
Benefits of using grow lights are these.
- Year-round plant growth is possible.
- Increased plant growth and yield
- Control over light intensity and spectrum
Different plant stages require different lighting.
In the germination stage, seeds need gentle light. The blue spectrum is preferred for the growth of seeds and transition into seedlings.
In the vegetative growing stage, plants need moderate to high light intensity. Blue and red spectrums should be emphasized. Blue light promotes leaf and stem development. Red light encourages leaf expansion and photosynthesis.
High levels of red light help in the flowering stage. This stimulates the production of flowers and fruits.
Conclusion
There are many factors we need to consider when maintaining an aquaponics system. Understanding the scientific background can give us a wider view and help us adapt to different situations.
Sources
https://gogreenaquaponics.com/blogs/news/what-is-aquaponics-biofilter
https://microbewiki.kenyon.edu/index.php/Nitrosomonas
https://gogreenaquaponics.com/blogs/news/the-importance-of-dissolved-oxygen-in-aquaponics
https://gogreenaquaponics.com/blogs/news/how-to-ventilate-your-indoor-aquaponics-system
https://gogreenaquaponics.com/blogs/news/the-effects-of-water-temperature-in-aquaponics