Looks like a bug/mite/mold type of thing.
I always start by spraying a soapy water solution on the leaves. 9 out 10 times it fixed the problem. Somebody said to use green dish soap not the other colors, there seems to be an issue with the other colors... I had green Palmolive on hand and it works...
all the best
Did you ever figure it out? I'm having the same issue and my numbers are almost identical to yours
I agree with Jim. Spider mites are tough little critters that can destroy plants in no time. If you have already sorted the Iron feed to the plants, look for small white (very small) or red (about a quarter of a pin head or smaller) spiders. When they multiply enough, you'll see the webbing on the plants.
My mixture of chilies garlic and soap (1 chilly, 1 garlic glove and 1 teaspoon of dish washing soap to a liter of water) will sort them out, but if the infestation is ugly, rather burn the plants. Do not put it on your compost heap!
Spray under the leaves as well. That's where they breed.
Let us know what you see.
On your numbers for the system. Why is ammonia high with no nitrites. How quick is the nitrification in your system?