The grow bed component of an aquatic system, is one of the most crucial. Choice of appropriate grow bed is often debated. Debates over what to use, repurpose, build, scavenge or buy for use in backyard aquaponics systems flourish.
Whether Deep Water Culture (DWC), Nutrient Filament Technique (NFT), similar non-media based, or media based, the bulk of grow bed selection criteria and general considerations are universal.
A Grow bed can be constructed a variety of materials. Whether purchased new, repurposed from another use, or built, care must be taken to ensure it fulfills certain criteria. At a minimum, the following should be considered. Grow bed design and construction must safely withstand stresses, weight, and pressures involved in containing water, media, plants, and worms. (All of these exert considerable weight and pressure on the structure of the grow bed). So too, must any stands or tables used to hold the grow bed. The material must be waterproof to avoid water leaks. It must be constructed of people, animal, and plant safe materials. It should allow a 1ft depth in grow media. If inside will its location support the weight and space requirements and will there be adequate light or light alternatives. If outside its material composition should be outdoor rated to withstand the elements and UV resistant to avoid breakdown form sunlight.
It’s material(s) must not leak unwanted chemicals into the water, or affect the pH of the water. Materials and construction should, ideally, block out sun, limited unwanted algae growth protecting plant roots in the media from excessive drying and overheating detrimental to plant roots. It should be UV stabilized to ensure that it will not degrade in the sunlight and leach chemicals into the system.
Grow Bed Materials
Commonly used materials in grow bed construction include certain plastics, wood, fiberglass and anything with suitable waterproof liners. Concrete and vermiculite are also be used but care needs to be taken to seal the concrete or vermiculite as they can negatively affect the water chemistry of the system.
The key concept is: Safe Materials….be sure all system components are plant, fish, and human safe.
As list of materials considered suitable to grow bed and tank construction is listed below.
Materials Considered Safe For Grow Bed/Tank Construction
Polypropylene - labeled PP
High Density Polyethylene - labeled HDPE
High Impact ABS (Hydroponic Grow Trays)
Stainless Steel barrels
EPDM or PVC (poly vinyl chloride) pond liner
(make sure its UV resistant and avoid fire retardant material)
Fiberglass tanks and grow beds
Rigid white PVC pipe and fittings, black flexible PVC tubing, some ABS
DO NOT use Copper – Its toxic to the fish
Grow Bed Depth and Size
An aquaponic grow bed must be sized appropriately and in complement of the fish tank volume. It must be able to provide adequate mechanical and biological filtration for the nutrient-rich water. For this reason, the grow bed and the fish tank sizes should be considered together to ensure a ratio between the two. A commonly used rule of thumb is a 1-1 ratio between tank and grow bed volume. A 1:3 tank to grow bed ratio will work quite well. Ratios of higher grow bed volumes to tank volume can readily be achieved with adjustments. The ratio and yield can both affected by adjustments to parts of the system. Some areas of possible adjustment would include the number of fish, how much they are fed, type, density, and nutritional requirements of plants, the nutritional content of the feed, volume of fish output, water volume and water flow rate, supplemental filtering and oxygenations techniques.
Appropriate grow bed depth is often a subject of discussion. Though there are always exceptions, there are some general experience guidelines about what has proven to be effective. A media based grow bed consists of three specific layers:
The surface layer - this layer should be approximately 2 in. (5 cm) in depth. It is above the waterline and should not be wet. The surface layer reduces loss of water through evaporation.
The root layer - this layer should be approximately 8 in. (20 cm) in depth. This is where roots will be growing and where most plant activity will take place. This is layer is normally regularly flooded and drained.
The solids layer - this bottom layer should be approximately 2 in. (5 cm) in depth. It is where solid waste will collect and will usually be constantly flooded with water.
These three layer combined yield a recognized suggested grow bed depth of approximately 12 in. (30 cm). It is important to remember that although 12 in depth has been proven itself highly effective, it is not the only depth that can be used. Experiences have demonstrated successful systems with grow bed depths as shallow as 4 in. (10 cm).
There is no required size, type, or construction of grow bed. Think safety first, but don‘t be afraid to be creative.
A Word About Stocking Densities
Aquatic life stocking densities is a function of the amount of feed processed y your aquatic life and the amount of resultant waste that can be filtered by the system. “Naturally balanced” systems relying on bacteria, worms, and aquatic life as the filtration system will support far less fish per tank than will systems incorporating more sophisticated filtration systems. Systems using only grow media, bacteria, and worms for mechanical and biological filtration support smaller densities of aquatic life but also require less labor and equipment than do systems incorporating additional filtration systems. Higher densities of aquatic life can be achieved, but do require greater effort and expense.
Experts and experienced aquaponists report that a “naturally balanced” system typically supports 1 lb of grown out fish for 5 to 10 gallons of water. In this case, a tank of 100 gallons would support 10 to 20 table sized fish. Introducing additional waste removal techniques with additional components such as charcoal filtrations, UV systems, clarifying, mineralizing, and degassing tanks, may support up to 1 lb of fish per 1.5 to 3 gallons of tank water. This latter goal also requires more intense management, greater labor, and expense in both setup and maintenance. Disposal of waste may also be problematic from a regulatory perspective. These considerations are very often the driving factors moving home and hobby aquaponists to “naturally balanced” media base growing systems.
A Word About Grow Bed Densities
Grow bed densities are basically a function how much waste is available for conversion to plant nutrients, how well the grow bed bio-filter can convert the waste to nutrients, and sunlight and water availability. The more nutrients available, the greater the ability to increase plant density. Remember that plants require sufficient sunlight and water as well. Even at traditional spacing, aquaponic systems can readily produce 5 to 8 times the amount of plant harvest as an equivelant traditional gardening (don’t forget the significant water usage reduction as well!).
The Relationship between Stocking Densities and Grow Bed Densities
High densities of both stock and grow bed densities can be achieved by introducing additional resources together with nutrition, areation, and water quality improvement techniques. In the absence of such techniques, in a media based system, a good ratio to work with is approximately 1 cubic foot (7.5 gallons) of grow bed per 1 pound of grown out fish. If one adopts the 5-10 gallons of aquatic life tank volume per 1 pound of fish, then roughly 1 cubic ft of aquatic life tank will support 1 cubic ft of grow bed. This suggests a 1:1 ratio of volumes between the aquatic life tank and the grow bed. Though many home-based aquaponists adhere to this ratio, with a bit of patience and effort, 1:2 ratios and greater may be achieved.
Choosing the System for You
Your system design will be a function your goals, your resources, and abilities. Remember. Aquaponic can be extremely enjoyable and rewarding. It is, however, it is an investment in an ongoing effort. Consider your initial investment, as well as your ongoing investments.
If you are new to gardening, stock management, or aquaponics, smaller may be the better way to start.
Though aquaponics is not difficult to master, it is far better to minimize risk and effort until you are certain your interest is more than passing. Expanding your system, or developing larger systems is easy and straight forward.
Carefully consider your investments and commitment as well as the characteristics and costs associated with each design type before you begin.