Magnesium Chlorosis
Check the soil first, to see if your output from the drain is about the same than your water input at pH 6.0 and EC 1.2. If the soil levels are correct, it’s because of to intense light. The plant is receiving more light than it can handle, Magnesium Chlorosis(*) is a symptom of a photoinhibition(*).
(*) Search to understand the process
Magnesium is the central ion surrounded and attached to 4 nitrogen atoms in the photosynthesis molecule. It gets used up at first during the photosynthesis process.