Found this below - seems your duckweed is sucking all the amonia and nitrogen from your system. Where do you grow your duckweed? Maybe seperate the duckwood nursery from your aquaponics? and only feed as required?
"When grown on swine wastewater, the duckweed Landoltia punctata was shown to remove 98% of the total nitrogen and ammonia content, and 94% of the total phosphorus. Considering these are the main agents of eutrophication, or nutrient pollution, the results are groundbreaking. If that wasn’t enough, L. punctata also improved levels of dissolved oxygen, crucial for the health of aquatic ecosystems."
thanks for the question - it triggered some memory on duckweed/AP system I've seen before and as I'm also busy with upscaling my system - considering duckweed as alternative/subliment feed. Please let us know if your system recover after removing the duckweed from ST. Some photo's appreciated to see the extend of coverage on your ST before and after.
Not sure why no visible poo though!