The following article is based on Wilson Lennard PhD’s Aquaponic System Design Parameters

Aquaponic fish to plant ratios, which is directly related to aquaponic feeding rate ratios, is the heart of a well performing unit. There are many approaches that attempt to define how many fish to place in your particular system and how much food to feed your system’s fish. Unfortunately many of these approaches are incorrect and have no real association with correct ratio determination methods for aquaponic systems. But there are two scientifically based approaches that will help get us on the right track. The UVI/Rakosy approach and the Aquaponic Solutions/Lennard approach. A third well based approach has also surfaced recently as well.

Defining Aquaponic Design Ratios

How do we size-up the two major components in a system, the fish and the plants? A common but partially unfounded method is to simply draw a relation between the amount of water or fish in the fish compartment and the amount of media in the growing beds. But if we were to look at what is actually occurs in an aquaponic system, we can get a better understanding of a more appropriate method.

We are aware of the aquaponic principal that the fish are fed, the fish produce wastes and this waste is used by plants for their growth. The amount of waste produced is in direct proportion to the amount of fish food consumed by the fish. The amount of plants that can be grown is proportional to the amount of nutrients available which in turn depends on the amount of waste produced by the fish. This in turn is dependent on how much food is fed to the fish.This is an uncomplicated circle of proportions. So the only real predictable ratio is based on the amount of fish feed entering the system related to the number of plants we grow. SO in actuality the ratios should be determined not by water nor media quantities but by the two major components of an aquaponic system… the fish and plant components.

Therefore, in actuality the ratios should be determined not so much by water nor media quantities but by the two major components of an aquaponic system… the fish and plant components.

Steps in Determining the Feeding Rate Ratio

Simply stated, the feeding rate ratio is how much fish food we need for the number of plant in the system. To get there, answer the following:

  1. How Many Plants you would like to produce
  2. How Much Area the plants need to grow
  3. How Much Fish Feed the fish need to produce enough nutrients for those plants
  4. What Amount of weight of fish are required to eat that much fish food.
  5. What Volume of Water that amount of fish need to be happy.
The fish-feed amount and the plants being grown is the Feeding Rate Ratio. As with any plant you grow, you need to give it nutrients. The nutrients are indirectly supplied by the fish food.

The UVI/Rakosy Approach

Dr. James Rackocy of the University of the Virgin Islands began some 30 years ago studying Aquaponic Systems.  His approach is measured in grams of fish feed/square meter of plant growing area per day. His bottom line using this formula is very general, which helps beginning operators to keep it simple. His bottom line formula is:

60 to 100g/m2/day

One of his most important observations was that fish have different requirements than plants as far as nutrition. The main differences are:

  1. Fish do not require the same amount of 2 important nutriens which plants do: Potassium- K and Calcium- Ca.
  2. Fish require a large amount of protein which once metabolized is Nitrogen- N and Phosphorus- P.
  3. Fish do not need other important macro-nutrients that plants require as well, Sulfur- S, Magnesium- Mg and Iron- Fe.

His findings showed that, most importantly, the fish wastes could never supply enough Calcium, Potassium nor Iron for  plant requirements.  His general approach is to feed elevated amounts of food to the fish so as to get as close as possible to plant requirements. This however, produced a dangerously high level of Nitrogen in the water for the fish. He eliminated these high levels of Nitrogen with an anaerobic denitrification technique.  Perhaps the most interesting practicality using his method, it produces both a high quantity of fish and plants.

The Aquaponic Solutions/Leannard Approach

The Aquaponic Solutions/Leannard Approach is based on lowering the fish quantity so the fish food required and its nitrogen produced was exactly equal to the amount of Nitrogen the plants need. Since the main produce of an aquaponic system is the vegetation not the fish, this would lead to a greater economical formula by saving initial construction and infrastructure costs for the fish.

His approach is highly specific to the type of fish and plants being grown. So there is no generalized ratio that can be given. But he does publish an example of his method using lettuce and Talapia. Dr. Leannard’s model states that:

1kilogram of fish food per day will support 1500 heads of lettuce grown on a rotational basis.
This relates to 16g/m2/day. (16gr of food for every m2 of normal ebb and flow aquaponic system.

The Difference in These Two Competing Models

The Leannard Approach is based on supplying only enough Nitrogen via the fish food but this requires adding to the system major elements. The Rackocy approach is based on giving the system all or most of the nutrients needed for the plants but eliminating excess nitrogen via denitrification. Denitrification via anaerobic microorganisms normally proceeds through a combination of the following intermediate forms:

Anaerobic Denitrification-   NO3 → NO2 → NO + N2O → N