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Here we go again, the myth makers are at it again. 

This time, it’s grow tank depth for raft systems. 

The suggestion is that grow tanks should be 300mm (12″) deep…..and some of the reasons being proferred for this depth include:

  • the more water in a system the greater its stability.
  • shallower tanks may heat up (or cool) down quicker.
  • shallower beds may become clogged with roots.
  • 12″ is a handy number for calculating with (my personal favourite).

Since all aquaponic growing systems are actually hydroponic systems, I consulted my copy of “Hydroponic Food Production” by Howard M. Resh PhD.

As it happens, water culture was one of the earliest hydroponic methods used in commercial crop production. 

Dr Resh describes an early method (circa 1970’s) that featured concrete grow troughs (painted with bituminous paint) that varied in width from 12″ – 42″ (300mm – 1650mm) and were 25′ – 100′ (7.5m to 33m) long……and 6” – 9″ (150mm – 225mm) deep.  The nutrient solution in the grow troughs was 4″ – 6″ (100mm – 150mm) deep.

Unlike the raft systems of today, these troughs did not use a styrofoam raft but rather supported a tray containing the plants.  Their roots were allowed to dangle in the nutrient solution.

Resh reports that, in 1977, the Japanese had about 300 hectares (about 750 acres of  greenhouse-based water culture.  The water troughs were made of rigid plastic and were typically 800mm wide, 3 metres long and were filled with nutrient solution to a depth of 150 – 200mm.

The troughs were fitted with a Styrofoam lid and the plants were inserted into holes in the lid.  This system grew mainly tomatoes, cucumbers and salad crops.

Dr Merle Jensen (University of Arizona) developed a raceway raft system during 1981 – 82.  The grow troughs that he used were 600mm (2′) wide, 200mm (8″) deep and 30m (90+ feet) long. 

 So, based on the above, it can be concluded that grow tanks do not need to be 300mm deep.  In fact, premised on the experience of the past, most of them…..and we’re talking hundreds of hectares here……were 150mm – 200mm deep.

Now, if you have 300mm (or deeper) grow tanks, don’t despair……they’ll work fine (just like 300mm grow beds do) but don’t let anyone tell you that they have to be 300mm.