I yanked out all my sickly lettuce today...murdered the whole lot.
Due to me not mixing the nutrients correctly and using tomato nutrient formula for lettuce, I ended up with some leaf edge "rot". They were growing great for around 33 days since seedling stage... But eventually all of them developed brown leaf edges.
Oddly the park choi has not shown any leaf edge issues.
Oh well. The tomatoes and cucumbers are doing good so far.
Starting over on more lettuce soon...WITH the correct nutrient mix.