Day 18: Plant looks healthy. I switched to a bucket with 4 liters of fluid today and increased the nutrient level to the plant's progress in growth. Now it has an EC of 1.7 with a PH of 5.8. The plant is still under 2x42W LED bars. Eventually when it flowers and starts to smell I'll move her outside on my balcony.
Day 22: The brown spots on the leaves means that something with the nutrients was messed up. Turns out, the PH was above 7, which is very different from the 5.8 I measured after the water change. Unfortunately I overcorrected at first, which caused the bottom leaves to crumble up the very same day. Eventually I diluted everything and brought the PH back to a normal level.