I was deeply interested in this, too, and did some major research into this. There also recently was a documentary on this very subject. The farmer (of both fish and crop) added no extra plant nutrients at all. It was far too costly to increase the nutes, kill some fish in the process, when he found that for his set-up it was more-or-less in balance anyway. I have used seaweed and fish emulsion to great effect, and recently I have also been trying diluted fresh human urine, which works surprisingly well (but a bit smelly for a day!)
