I’ve been doing some research on the aquaponics systems design. I found some great information and sources that I wanted to share with you. I’ve found a lot of websites and a few forums about aquaponics that have provided a lot of value, but I think the best source that I have found for calculating my needs is from Dr. Wilson Lennard. He has years of practical and technical experience with fish, hydroponics, and aquaponics. The website www.aquaponic.com.au contains a simple to use excel spreadsheet calculator developed by Dr. Lennard for determining what ratio and sizes of fish/grow bed/biofilter you need. The calculator can be downloaded here:
A grow bed depth of about 12 inches (300 mm) is used by many and seems to be in wide consensus as the appropriate size for most aquaponic plant roots. The surface area of the grow bed is not as clear cut.
A common rule of thumb on grow beds is to have a 1:1 or 2:1 ratio of grow bed:fish tank. This refers to the volume of the container (without any of the grow bed media inside). I’ve seen that this isn’t really followed, however, in a lot of cases. For example many IBC (tote) containers have been used by cutting the top 12″ inches off for the grow bed. This leaves approximately 250 gal to grow the fish, and 25 gallons for the grow bed. This would be a ratio of 1:10!
In reality, the ability of the grow bed to efficiently filter and grow beneficial bacteria is a balance with the amount fish in the tank, they amount of food provided to the fish, the surface area of the media bed, the type of media in the media bed, how often the water is recirculated, and the volume of water in the fish tank. Dr. Lennard gives a recommended fish density of 5 to 15 kg of fish per cubic meter of water. This means that for a pound of fish (that is a pretty large fish) you will need about 8 to 24 gallons of water in the fish tank. Going at a higher density would likely require more oxygen to be supplied to the fish tank.
Below is a graphical representation of the ratios found in Dr. Lennard’s spreadsheet.
Notice that the ability to filter and mineralize the fish waste in the grow bed is dependent on the amount of food that is fed to the fish. Dr. Lennard recommends feeding food that equals about 1% of the fish weight per day.
The volume in the fish tank assumes that there is a sump also. The sump holds additional water that allows the fish tank to have a constant water level. If this isn’t the case, you’ll need to increase the volume of water to account for the water that will fill the grow bed(s) during the ebb and flow cycle. If you are designing a deep water system (raft system) or Nutrient Film System you will need to have an appropriately sized biofilter.
I found the calculation spreadsheet to be very helpful, and I recommend you take a look.
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