Just some questions that will help everyone reading this thread... how old is the system? what is the source of your water? what is the PH? what have you already tried?
The system is still young, and in my experience young systems usually have deficiencies. It looks like iron to me which you can get yourself some Maxicrop Liquid Seaweed and spray it on the foliage. It may take some time for the biology in your media beds to become robust enough to mineralize the fish poo. The Nitrates are eazy to get - the rest takes time.
Not an iron deficiency. More likely Calcium and Magnesium. Water from a desal plant is going to be very soft.....
Having tried dwc twice, the most recent still on going for over a year, I find no advantage and many disadvantages for a BY system so my question is "if you are having great luck with media F&D why not do as I am about to do for the second and last time, change that bed over to media, F&D?
Now if yours is a large commercial system, best of luck and never mind as a rotating raft system for commercial growing is hard to beat. For a BY homestead system 24x24GH, 2500gal with trout in our case, I would stick to media F&D. Next to no input or monitoring on a daily basis (I can't even recall the last time I got out the test kit) unlike a commercial system that requires constant monitoring and a constant battle with root rot but it is a full time endeavor by comparison to a BY 5 minutes per day involvement (not counting harvests of course but that is the reward). Just my 3+ years 2 cents worth.
We are having a big trout family dinner party tomorrow for our daughter's birthday so that will be over the usual 5 min involvement. Yum!
I agree with Scott; likely a calcium deficiency.
If you're looking for a Cal Mag product, www.kelp4less.com has a Cal-Mag Organic product that contains no nitrogen (according to them anyway). Since it's in it's organic form, it's a little more "slow release", but they said it works well for soiless. I haven't found a way to accurately dose with it yet though.
Likely more than one element deficient (Ca, Mg, Fe & others) but I would target Fe and Mg deficiencies first--with iron chelate and MgSO4 (epsom salts). In hydroponics you would target about 40 ppm Mg in your solution. In aquaponics I don't know that you should set a target. I have been struggling with Mg and Fe deficiencies in my new system. I have about 350 gal total AP solution.
I have added 78 grams MgSO4 three times about one week apart. 78 grams is the quantity I use in 50 gal of solution in my hydroponic system.Mg is a mobile nutrient--so you may be able to bring your plants out of this deficiency.Use a chelated iron. I have also been adding chelate iron at 5 grams, again the amount I use for 50 gal of hydroponic solution. The yellow crook-neck squash (photo attached) is coming around. Note the interveinal chlorosis evident on the older leaves.