I started a conversation on another forum about aquaponics. most people were intrigued but one member -who I really do respect- came back with this response.
I have permission to quote him word for word and I ask that I can quote your answer back to him.
This is what he said
I have to say I've not looked into Aquaponics too deeply, mainly because I can't see it working.
On the other hand I have four Hydroponics systems operating here, with the first one running continuously now for 7 years, so I do know a bit about hydroponics.
It does sound a perfect system, with plants cleaning the water for the benefit of the fish and the fish "fertilising" the water for the benefit of the plants. But I'm pretty sure it's not that simple.
The only system I can see working is where the foul water is "watered" onto plants growing in soil, but not growing plants in inert media with the water recycled.
For instance vegetable plants need a wide range of trace elements and brassicas (for instance) need high levels of nitrates, which would not be produced in high enough concentrations by a small quantity of fish. Which means either the plants would be stunted or you would have to add extra, but ONLY THE EXACT AMOUNT the plants would absorb at that time, or the fish would suffer from high nitrate levels and the fish tank would become a pea soup of algae.
When I grow cauliflowers in my hydro I have to add a largish quantity of magnesium (Epsom Salts) once a week to prevent leaf tip burn which I'm sure would be detrimental to fish. The Epsom Salts I add to the tank is enough for 7 days, if there were fish to consider I would have to add it daily in exact quantities, the same as the nitrates.
So I'm sure if totally necessary it could be done, but it would be far too complicated and time consuming for me, and as I'm not that keen on fresh water fish anyway, I'll just grow my fruit and veg on their own.
HI Paul. I'm laughing because that is exactly the kind of skepticism I had when I first got into aquaponics. I came from a 7 year background in Hydroponics via running the plant grow lab at AeroGrow, international in the development of the AeroGarden. I actually have my name on some patents for hydroponic nutrients, and I was convinced that there was NO way that it could be as simple as aquaponic practitioners said it was. But what I had lost site of was that a media-based aquaponic system is actually closer to a soil garden than a hydroponic garden. Because the "nutrients" are natural animal waste there is no need to "balance" them. And just as you don't worry about supplementing magnesium in your soil garden (or at least I don't...) you generally don't need to worry about that in AP. We grow a wide variety of crops, and never replace the fish water. We just make sure the pH is kept in proper range, and that we bring it up when necessary with natural calcium and potassium carbonate. The only thing we find we need to regularly supplement, aside from adjusting pH, is iron. The maintenance is far less than hydroponics because we don't ever replace the water in the system, which hydro guys need to do every few weeks.
Believe me, and thousands of others who are successfully growing organic fish and veggies across the world - it really works!
Seva Tokarev wrote:
Still, what about trace elements? Are they supposed to come from fish feed? Does it impose additional requirement on the latter?
Thanks for your kind words.
In general the answer is "yes, the trace minerals come from the fish feed", but sometimes deficiencies do occur and if that is the case we recommend a rock dust product called Aquaponic Elements. It works like a tea bag that you place under the water-in spout in your grow beds and it slowly releases trace minerals into your system (without the extra nitrogen and sodium that AP systems don't need).
Also, magnesium sulfate, or "epsom salt", is quite safe for fish. There are plenty of successful aquaponic growers that use it in their systems; I've also used it in mine. Like Sylvia said, trace elements are mostly provided by the fish food, but if it's still a concern, you can use water soluble kelp powder to provide what you need as well as give you some extra potassium.
"There is nothing, Sir, too little for so little a creature as man. It is by studying little things that we attain the great art of having as little misery and as much happiness as possible." - Samuel Johnson
Companion Planting Guide by World Permaculture Association