Aquaponics: A Guide to Integrated Fish and Plant Production
How aquaponics combines fish farming with hydroponics: fish waste is nitrified into plant nutrients, plants clean the water, plus components, ratios, pH balance, and common fish and crops.
Overview
Aquaponics is the integration of aquaculture, raising fish in tanks, with hydroponics, growing plants without soil. The two are linked in a single recirculating loop: nutrient-rich water from the fish passes to the plants, which take up the nutrients and return cleaner water to the fish. FAO presents aquaponics as a method that combines the nitrogen cycle of a recirculating fish system with soilless plant production.
How it works: the nitrogen cycle
Fish excrete waste mainly as ammonia, which is toxic to them. Nitrifying bacteria convert this ammonia first to nitrite and then to nitrate, the form of nitrogen that plants readily absorb as fertilizer. By taking up nitrate and other dissolved nutrients, the plants help keep the water suitable for the fish. The bacterial biofilter is therefore the link that makes the shared loop work, the same nitrification used in recirculating aquaculture systems.
Components
- Fish tank: holds the fish and is the source of nutrients.
- Biofilter: surface area where nitrifying bacteria convert ammonia to nitrate (media beds can provide this).
- Hydroponic units (one of three main designs below) where plants grow.
- Sump and pump: collect and circulate water around the loop.
- Optional solids removal: a settling component to take out fish solids.
Hydroponic methods
- Media bed: grow beds filled with gravel or expanded clay, flooded and drained; the media also acts as biofilter.
- Nutrient film technique (NFT): a thin film of water flows along channels past the plant roots.
- Deep water culture (DWC), or raft: plants sit on floating rafts with roots suspended in the water.
Balancing fish, feed and plants
The system is balanced through the feed-rate ratio, the amount of fish feed added per unit of plant growing area each day. FAO gives a guideline of roughly 40–50 g of feed per square metre of plant area per day for leafy vegetables and about 50–80 g per square metre per day for fruiting vegetables. During grow-out, fish are typically fed about 1–2 percent of their body weight per day. Too little feed starves the plants of nutrients; too much overloads the biofilter and raises ammonia.
Water quality and the pH compromise
Fish, plants and nitrifying bacteria have different ideal pH ranges, so aquaponics is run at a compromise. FAO guidance is to keep the pH between about 6 and 7, adding small amounts of a base or buffer when nitrification drives the pH down toward 6. Adequate dissolved oxygen is needed for the fish, the plant roots and the bacteria, and higher fish densities require more aeration to keep oxygen stable.
Common fish and crops
Nile tilapia (Oreochromis niloticus) is the most widely used aquaponic fish because it tolerates a broad range of conditions, and carp, catfish and ornamental species such as koi and goldfish are also used. Suitable crops include leafy greens such as lettuce and kale, herbs such as basil and mint, and fruiting vegetables such as tomatoes, peppers and cucumbers; leafy greens and herbs are easiest because of their lower nutrient demand.