Youre having nutrient deficiency, probably nitrogen. Nitrogen is a mobile element, so the older growth will turn yellow when there's a lack of it for new growth.
But I think that your main problem is watering.
25L is a big pot, and youre also growing in coco which do not hold a lot of water for a long time.
I would probably water at least 4-5 liters per day.
What is the EC of the water youre using? Is it high? I notice you go just 0.66ml from the Flora series and I wonder why.
Happy growing mate! 💪