I have an aquaponics install that is currently "fishless" in that I am in the final phases of getting it to cycle ammonia into nitrogen. After transplanting a bunch of sickly plants started during cold weather, I was encouraged when even sickly bean plants recovered and grew thick, deep emerald green leaves.
Soon, though, I noted the same spotty yellowy mottling was coming back. I thought this was fungus and following the advice of a local hydroponic store sprayed a $14.00 fruit smoothy fungicide all over.
This did not correct the problem.
My wife Gemey did some research and we dialed back the frequency of the flood/fill cycle from 15 minutes every hour to 15 every 90 minutes. No improvement after several days.
More research yielded the theory that we are deficient in potassium. Murry Hallam acknowledged that AQP systems would be potassium poor and that he supplemented his tanks. I posted images of the plant leaves - this is affecting tomatoes, squash, beans and peppers. The strawberries and herbs so far are doing OK.
I added 200ml of Sweet and Heavy "Bloom" fertilizer. (Earth Juice) which has a slug of potassium. 24 hours later, my wife thinks the plants look happier.
I observe that new leaves - a few days old, are wonderful and healthy looking. It's older leaves that are curling and looking burnt.
I burned about half a bushel of hardwood (Oak and maple) twigs and added a cup of ash to a gallon of water and let it sit for 10 hours. I strained the water after it settled into a spray bottle to make a potassium foliar spray. I sprayed two plants that looked like they were about to go bad, and a plant that was otherwise healthy to see if this may act as an emergency fix to halt the problem, given that organic fertilizers take a few days to become available to plants.