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Don’t you forget about fish



An expert in aquaponics, Dr. Wilson Lennard, Phd., remarked that many aquaponic farmers tend to neglect the fish. Most are inclined to think that so long as the fish are alive, then they’re probably okay. This kind of thinking, unfortunately, is far from the ideals of proper fish husbandry. Worse, it forgets that if the fish suffers, the aquaponic system — and everything in it — also suffers.

The signs of i

mproper fish husbandry can result to fish death or, for the time being, a gradual decline in health due to depressed immunity. Consequences also include poor growth, erratic appetite, and odd behavior such as listing or gasping for air on the water surface. Much, if not all, of these problems can be blamed on bad water. This aspect of running an aquaponic system is often neglected until the fish start to die off.

To avoid these from happening, the ideal compromise water parameters for the fish (ex. tilapia) and the plant components of the aquaponics system must be maintained: temperature (18-30 °C), pH (6.8-7), ammonia (0ppm), nitrites (0ppm), nitrates (5-150ppm), and dissolved oxygen (5-6mg/L). These indicators are also affected by water volume, shape and size of the fish tank, stocking density, feeding rate and water turnover rate per hour.

THESE healthy creatures require ideal water parameters.


THE fish in the holding tank observed by this writer’s nephew.


For example, it is generally recommended that beginners use an IBC tank that roughly has 1000 liters of water capacity. This volume will give a more stable water environment, reduces the risk of sudden swings, especially for temperature and pH.

As for shape, a circular tank is preferred with a depth to diameter ratio of 1:3 (some choose 1:5, if extra space is available). Then, let the incoming flow run parallel to the tank’s circumference. This pushes the water to flow in circles leading to the center where solids (fish poo) accumulate and can be easily drained out.

Further, this kind of water movement, coupled with the circular tank shape, lets the fish continuously swim to without having to make drastic turns at corners. Interestingly, a properly exercised and healthy fish consumes less oxygen, probably through efficient use.

Stocking density is also important. For aquaponics 20kg per 1000 liters, or 40 half kilo fish per 1000 liters, is usually the benchmark. Remember, however, that more fish means faster oxygen depletion and faster buildup of toxic ammonia and nitrites. Hence, besides proper stocking density and getting rid of accumulated solids, regular water turnover is important.

The usual prescription is to have an hourly turnover of the entire water volume of the tank. Some go down to as low as half of that per hour. Note the rule of thumb though, as stocking density further increases (ex. to 50kg per 1000 liters of water), the flow rate should be closer to an hourly turnover of the entire fish tank’s volume.

Adding aeration is useful too, but has limits. Some push stocking density higher under the false understanding that more air automatically means more oxygen. It isn’t so. The air delivered by the pump through air stones comes from the atmosphere, which only has an oxygen content of 20 percent. Thus, oxygen delivery of air pumps cannot — by rule of science — go beyond that. Also, increased dissolved oxygen in the water is temperature dependent.

There are many considerations for the fish component of aquaponics. We have just scratched the surface. To test if the takeaway gained some traction, feel free to fill in the blanks: “don’t you forget about ________, don’t, don’t, don’t, don’t!