Recirculating Hydroponic System with No Water Change

by Kaushik Patel
(Montreal, Canada)

I am going to try DWC for the first time, but with Zilli Tilapia's and bioballs, which is use to convert ammonia to nitrates. Basically, it is an aquaponic system which recirculates the water. Now, if I add nutrients to my system, will I have to change the water every so often? And /or will my Tilapias provide enough nutrients to my system (which I don't think will happen). Also, I will be having different stages at one time....will having a high ppm hurt my early stage roots? Or do I have to have different systems for different stages? Thank you Guru for lots of info you provide!

Answer: Kaushik- this type of recirculating hydroponic system, using aquaponics to fish-farm, is a little outside my expertise. Anyone who has ever managed an aquarium can tell you that high nitrate levels will begin to kill your fish. I have talked with at least two other visitors who are trying aquaponics, and this seems to be the biggest sticking point in the whole operation (balancing the Nitrate requirements of the crop with the Nitrate requirements of your fish).

Most crops you would like to grow require Nitrate levels that are too high to be healthy for your fish. The fish, on the other hand, would prefer very low levels of Nitrates, but will tolerate medium-low levels without too much of a problem. If you are serious about bringing this type of recirculating system into perpetual balance and harvest, this leaves you with a handful of crop choices. The most common ones I have seen are micro-greens, lettuce, spinach, and herbs that produce vegetatively....which include rosemary, basil, oregano, mint, sage, thyme, and others.

Running a small worm farm is one option I have discussed with another visitor. Tea made worm castings can add Nitrogen, Phosphorus, and Potassium to the water in your system, but does not seem to bother the fish as much as other Nitrate sources. Also, the redworms can be used as a supplemental food source for the fish.

Having a high ppm will hurt your fish more than it will hurt your younger plants. Be sure to do a little more research into what levels of Nitrate (and other plant nutrients) your fish will tolerate before deciding to add any additional fertilizer to this type of system. In a recirculating hydroponic system, growing the crops I have mentioned above, a balance can be achieved. Certain Nitrate levels can be maintained that do not affect the fish negatively, but which allow for several crops at different stages to be grown and harvested simultaneously.

One of my other visitors was actually using 1 1/2 inch thick styrofoam, similar to the lids made for this system in order to float each different crop as he added them to his system. He was also farming Tilapia, using bioballs, and was growing many different lettuce and micro-green varieties in a perpetual, recirculating fashion without any water changes. I hope this gives you some good ideas Kaushik. I would love to hear how it turns out....and Happy Growing!