Most of Southern Africa is blessed with hot summers and bitterly cold winters, and it is against this backdrop that we farm fish and plants in an integrated manner in our Aquaponics systems.  It is important to remember that each of the 3 key Aquaponics ingredients – bacteria, fish and plants – has a peak performance temperature at which metabolism occurs optimally.  The bacteria function best at around 27?C, whereas different fish and plant species each have their own preferred temperature.  If you rear tilapia or catfish they should be kept at around 28?C which means that you would be using warm climate plants such as cucumbers, herbs, tomato, peppers and the like, unless you have a decoupled system.  Those farming under cooler conditions with trout as the fish species then you would typically be producing peas, lettuce and watercress, and be maintaining temperatures around 18?C, unless again you have a decoupled system.  Under these cooler conditions the bacteria are far slower and more are therefore required to convert the toxic ammonia to plant food, necessitating the use of a higher growbed to fish ratio under cool water conditions.
 
We started off by saying that our climate is distinctly seasonal, yet the crops have a constant temperature requirement.  I do not recommend the option of having tilapia for summer and trout for winter due to the long growing season they each require on the one hand combined with the desire to harvest and sell continuously on the other hand.  The option does exist to allow the temperature to drift a little across the seasons, compromising the fish growth for part of the year and changing your plants to suit the season.  That is, assuming this fits the requirements of the market you supply.
 
Keeping the water cool in summer is best achieved by shading the fish (they do not require bright sunlight) and opening the doors and vents of the greenhouse tunnel.  Under extreme conditions it is possible to use geothermal cooling or evaporative cooling to prevent the temperature of the air or water respectively from exceeding the desired levels.
  
Given that it is currently mid-July, we are at present acutely aware of how cold winter can be and the need for a method of maintaining water temperatures.  The greenhouse tunnel itself does passively generate heat during the day and reduce the rate of heat loss at night, but this alone is generally inadequate to maintain the required temperatures for optimal plant and fish growth.  Active heating can be done in small systems using direct heat (geyser element and a good quality thermostat), but should be combined with insulation, such as a greenhouse, to avoid excessive heat loss at night.  In larger systems you can use a boiler in parallel with the aquaponics system to heat a body of water that is used as a heat source to in turn accurately heat the aquaponics water via thermostatic control.  This requires the availability of a fuel source (wood, coal) and is messy but can be very cost effective if a cheap fuel is available.  Most commonly we use heat pumps to efficiently convert electrical energy into heat.  These plug-and-play units are simple to operate and reliable, but do not work well in sub-zero temperatures.
 
This brings us full circle back to the importance of considering the climate when selecting the crops you will produce so that the design of the infrastructure and method you will use for heating or cooling can be optimised for the combination of plants and fish you wish to farm.  Alternatively, use a decoupled system which simplifies maintaining ideal conditions for each crop.
 
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Generous vents on the ends of tunnels are effective at cooling