Have you tested your water? What are your levels at?
the fish density does seem low for such a large grow bed. also depends on how many heavy feeding plants you have as well. any fruiting plants will take way more nitrogen than leafy greens will. i have had similar issues and added very small amounts of blood meal to base of affected plants with decent results as a quick fix until my fish density can be raised. have to be extremely careful though because the blood meal with spike ammonia, then nitrite, then nitrates so have to monitor carefully and it takes a few days in my findings
That seems obvious I suppose. I have a kit and this will be my next step.
Roland, test for nitrates. Nitrates are your plant's source of nitrogen. Meanwhile, if you could post a picture of your basil, that would be great. There are lots of deficiency symptoms that cause yellowing in the leaves; it all depends on the kind of yellowing and where in the plant it is. Are the leaves yellowing, but the veins staying green? Did it start in the new leaf growth or the old leaf growth?
Didn't have a test for nitrates, only nitrites. That theory about the pH sounds right. I will buy the liquid that lowers pH.