Ah, I used gravity return on my system like this. I'm guessing you used a return pump? Mine never got a chance to go stagnant when the pump was running, the buckets were always overflowing back into the rez. Never did like that two pump system for that reason.
I've worked with rocky on his other thread. My recommendation then and now was to get some oxygen into the buckets. He uses a pump-fill, gravity-drain setup which means no recirc, just a static fill.
I missed the bit about only 3 fills a day.
So i agree with both under- and overwatering: not enough fills, and with the pellets in the bucket and that fibrous collar, I think the air in the buckets gets stagnant.
Since the plants are in distress, i recommend aggressive action: 1) at least double the number of fill cycles, and 2) get an airstone into the bottom of each pot to displace the motionless air in there! cn